diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6b054b1..5a5283b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -273,6 +273,19 @@
 - CI keeps zlib-ng building and tested; the arm64 job no longer needs cmake.
 
 ### Correctness
+- **Files with a user block** (`h5py.File(..., userblock_size=N)`, `h5jam`;
+  the superblock at 512, 1024, …) could not be read: every address in the
+  file is relative to the superblock, but it was applied from byte 0
+  (`InvalidObjectHeaderVersion` on the root group). `File` (mmap, buffered,
+  `from_bytes`), `MmapFile`, `LazyFile`, `AsyncHDF5File`, the VOL readers,
+  the HNSW loader and external VDS sources now view the file from the
+  superblock on, using the signature's position as the base address as
+  libhdf5 does; `user_block_size()` reports the user block (h5py's
+  `userblock_size`), and `as_bytes()` returns the bytes from the superblock
+  on. **Breaking (format crate):** `Superblock::parse` refuses a non-zero
+  signature offset with `FormatError::UserBlockNotStripped`, since the
+  addresses it returns would be applied to the wrong bytes; pass the slice
+  from `signature::split_user_block` (new) and parse at offset 0.
 - `clawhdf5-format` reader: version-1 shared messages (HDF5 1.6-era files,
   e.g. a dataset using a committed datatype in libhdf5's `tcompound.h5`)
   read the heap-offset field of the embedded symbol-table entry as the
diff --git a/crates/clawhdf5-ann/src/hnsw.rs b/crates/clawhdf5-ann/src/hnsw.rs
index a526bee..a725b3f 100644
--- a/crates/clawhdf5-ann/src/hnsw.rs
+++ b/crates/clawhdf5-ann/src/hnsw.rs
@@ -13,7 +13,7 @@ use clawhdf5_format::filter_pipeline::FilterPipeline;
 use clawhdf5_format::group_v2::resolve_path_any;
 use clawhdf5_format::message_type::MessageType;
 use clawhdf5_format::object_header::ObjectHeader;
-use clawhdf5_format::signature::find_signature;
+use clawhdf5_format::signature::split_user_block;
 use clawhdf5_format::superblock::Superblock;
 use clawhdf5_io::FileWriter as IoFileWriter;
 
@@ -861,8 +861,9 @@ impl HnswIndex {
     /// The HDF5 data must contain the `/ann/vectors`, `/ann/graph_layer_*`,
     /// and `/ann/config` datasets as produced by [`to_hdf5_bytes`].
     pub fn load_from_hdf5(data: &[u8]) -> Result<Self, FormatError> {
-        let sig_offset = find_signature(data)?;
-        let sb = Superblock::parse(data, sig_offset)?;
+        // Addresses are relative to the superblock: skip any user block.
+        let (_, data) = split_user_block(data)?;
+        let sb = Superblock::parse(data, 0)?;
 
         // Read config dataset and its attributes
         let config_attrs = read_dataset_attrs(data, &sb, "ann/config")?;
diff --git a/crates/clawhdf5-format/src/data_read.rs b/crates/clawhdf5-format/src/data_read.rs
index 82e6544..8d3b16e 100644
--- a/crates/clawhdf5-format/src/data_read.rs
+++ b/crates/clawhdf5-format/src/data_read.rs
@@ -575,11 +575,13 @@ fn read_named_dataset_raw(
     use crate::group_v2::resolve_path_any;
     use crate::message_type::MessageType;
     use crate::object_header::ObjectHeader;
-    use crate::signature::find_signature;
+    use crate::signature::split_user_block;
     use crate::superblock::Superblock;
 
-    let sig = find_signature(file_data)?;
-    let sb = Superblock::parse(file_data, sig)?;
+    // An external source file is handed over whole, user block included;
+    // its addresses are relative to its superblock.
+    let (_, file_data) = split_user_block(file_data)?;
+    let sb = Superblock::parse(file_data, 0)?;
     let addr = resolve_path_any(file_data, &sb, path)?;
     let hdr = ObjectHeader::parse(file_data, addr as usize, sb.offset_size, sb.length_size)?;
 
diff --git a/crates/clawhdf5-format/src/error.rs b/crates/clawhdf5-format/src/error.rs
index 7ae57f4..a54ca72 100644
--- a/crates/clawhdf5-format/src/error.rs
+++ b/crates/clawhdf5-format/src/error.rs
@@ -120,6 +120,11 @@ pub enum FormatError {
     /// A shared-message reference points at an object header that holds no
     /// (unshared) message of the referenced type (raw message type id).
     SharedMessageTargetMissing(u16),
+    /// A superblock was parsed at a non-zero offset of the buffer (the file
+    /// has a user block of this many bytes). HDF5 addresses are relative to
+    /// the superblock, so the buffer must start there: see
+    /// `signature::split_user_block`.
+    UserBlockNotStripped(u64),
     /// A selection does not fit the dataset it was applied to (wrong rank, or
     /// it reaches past a dimension's extent).
     SelectionOutOfBounds(String),
@@ -342,6 +347,11 @@ impl fmt::Display for FormatError {
             FormatError::SelectionOutOfBounds(msg) => {
                 write!(f, "selection out of bounds: {msg}")
             }
+            FormatError::UserBlockNotStripped(n) => write!(
+                f,
+                "file has a {n}-byte user block: parse the bytes from the superblock on \
+                 (signature::split_user_block)"
+            ),
             FormatError::SharedMessageTargetMissing(t) => write!(
                 f,
                 "shared message reference points at an object header with no message of type \
diff --git a/crates/clawhdf5-format/src/lib.rs b/crates/clawhdf5-format/src/lib.rs
index 4a6c2a1..ffed10f 100644
--- a/crates/clawhdf5-format/src/lib.rs
+++ b/crates/clawhdf5-format/src/lib.rs
@@ -26,12 +26,13 @@
 //! use clawhdf5_format::{signature, superblock, object_header, group_v2,
 //!     datatype, dataspace, data_layout, data_read, message_type::MessageType};
 //!
-//! let file_data = std::fs::read("output.h5").unwrap();
-//! let sig = signature::find_signature(&file_data).unwrap();
-//! let sb  = superblock::Superblock::parse(&file_data, sig).unwrap();
-//! let addr = group_v2::resolve_path_any(&file_data, &sb, "data").unwrap();
+//! let bytes = std::fs::read("output.h5").unwrap();
+//! // Addresses are relative to the superblock: skip any user block.
+//! let (_user_block, file_data) = signature::split_user_block(&bytes).unwrap();
+//! let sb  = superblock::Superblock::parse(file_data, 0).unwrap();
+//! let addr = group_v2::resolve_path_any(file_data, &sb, "data").unwrap();
 //! let hdr = object_header::ObjectHeader::parse(
-//!     &file_data, addr as usize, sb.offset_size, sb.length_size).unwrap();
+//!     file_data, addr as usize, sb.offset_size, sb.length_size).unwrap();
 //! ```
 //!
 //! # Features
diff --git a/crates/clawhdf5-format/src/shared_message.rs b/crates/clawhdf5-format/src/shared_message.rs
index a77edc1..f760021 100644
--- a/crates/clawhdf5-format/src/shared_message.rs
+++ b/crates/clawhdf5-format/src/shared_message.rs
@@ -408,8 +408,8 @@ pub fn load_sohm_table(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Option<SohmTable>, FormatError> {
-    let sig = crate::signature::find_signature(file_data)?;
-    let sb = crate::superblock::Superblock::parse(file_data, sig)?;
+    // `file_data` starts at the superblock (see `signature::split_user_block`).
+    let sb = crate::superblock::Superblock::parse(file_data, 0)?;
     let Some(ext_addr) = sb
         .superblock_extension_address
         .filter(|&a| !is_undefined(a, offset_size))
diff --git a/crates/clawhdf5-format/src/signature.rs b/crates/clawhdf5-format/src/signature.rs
index 27d5e75..600b650 100644
--- a/crates/clawhdf5-format/src/signature.rs
+++ b/crates/clawhdf5-format/src/signature.rs
@@ -11,6 +11,16 @@ pub const HDF5_SIGNATURE: [u8; 8] = [0x89, b'H', b'D', b'F', b'\r', b'\n', 0x1A,
 /// (powers of two starting at 512, plus offset 0).
 ///
 /// Returns the byte offset where the signature was found.
+///
+/// A non-zero offset means the file starts with a *user block*, and every
+/// address inside the file is relative to the superblock's position, not to
+/// byte 0 (libhdf5 uses the signature's position as the base address even
+/// when the stored base-address field disagrees). The parsers in this crate
+/// take addresses as indices into `file_data`, so they must be handed the
+/// bytes from the signature on — use [`split_user_block`]. [`Superblock::parse`]
+/// refuses a non-zero offset for this reason.
+///
+/// [`Superblock::parse`]: crate::superblock::Superblock::parse
 pub fn find_signature(data: &[u8]) -> Result<usize, FormatError> {
     // Check offset 0
     if data.len() >= 8 && data[..8] == HDF5_SIGNATURE {
@@ -29,6 +39,17 @@ pub fn find_signature(data: &[u8]) -> Result<usize, FormatError> {
     Err(FormatError::SignatureNotFound)
 }
 
+/// Split a file into its user block and its HDF5 bytes.
+///
+/// Returns `(user_block, hdf5)`: `user_block` is everything before the
+/// superblock signature (empty for most files) and `hdf5` is the rest, in
+/// which every HDF5 address is a plain index. Pass `hdf5` as `file_data` to
+/// every parser in this crate, and parse the superblock at offset 0 of it.
+pub fn split_user_block(data: &[u8]) -> Result<(&[u8], &[u8]), FormatError> {
+    let offset = find_signature(data)?;
+    Ok(data.split_at(offset))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -88,6 +109,21 @@ mod tests {
         assert_eq!(find_signature(&data), Err(FormatError::SignatureNotFound));
     }
 
+    #[test]
+    fn split_user_block_rebases_at_the_signature() {
+        let mut data = vec![7u8; 1024];
+        data[512..520].copy_from_slice(&HDF5_SIGNATURE);
+        let (ub, hdf5) = split_user_block(&data).unwrap();
+        assert_eq!(ub.len(), 512);
+        assert_eq!(hdf5.len(), 512);
+        assert_eq!(&hdf5[..8], &HDF5_SIGNATURE);
+
+        data[..8].copy_from_slice(&HDF5_SIGNATURE);
+        let (ub, hdf5) = split_user_block(&data).unwrap();
+        assert!(ub.is_empty());
+        assert_eq!(hdf5.len(), 1024);
+    }
+
     #[test]
     fn signature_prefers_earliest() {
         // Signature at both 0 and 512, should return 0
diff --git a/crates/clawhdf5-format/src/superblock.rs b/crates/clawhdf5-format/src/superblock.rs
index e971495..d2ec4d6 100644
--- a/crates/clawhdf5-format/src/superblock.rs
+++ b/crates/clawhdf5-format/src/superblock.rs
@@ -174,8 +174,18 @@ impl Superblock {
 
     /// Parse a superblock from `data` starting at `signature_offset`.
     ///
-    /// The signature must be present at the given offset.
+    /// The signature must be present at the given offset, and that offset
+    /// must be 0: every address in an HDF5 file is relative to the
+    /// superblock, so when a file has a user block (signature at 512, 1024,
+    /// …) the caller must pass the bytes from the signature on — see
+    /// [`crate::signature::split_user_block`] — and use that slice as
+    /// `file_data` everywhere. A non-zero offset is refused with
+    /// [`FormatError::UserBlockNotStripped`] because the addresses in the
+    /// returned superblock would otherwise be applied to the wrong bytes.
     pub fn parse(data: &[u8], signature_offset: usize) -> Result<Superblock, FormatError> {
+        if signature_offset != 0 {
+            return Err(FormatError::UserBlockNotStripped(signature_offset as u64));
+        }
         let d = data
             .get(signature_offset..)
             .ok_or(FormatError::UnexpectedEof {
@@ -676,7 +686,16 @@ mod tests {
         let mut data = vec![0u8; 1024];
         let v0 = build_v0_bytes(8);
         data[512..512 + v0.len()].copy_from_slice(&v0);
-        let sb = Superblock::parse(&data, 512).unwrap();
+        // Addresses are relative to the superblock, so parsing in place
+        // (where they would be applied to the whole buffer) is refused...
+        assert_eq!(
+            Superblock::parse(&data, 512),
+            Err(FormatError::UserBlockNotStripped(512))
+        );
+        // ...and the caller parses the bytes from the signature on.
+        let (ub, hdf5) = crate::signature::split_user_block(&data).unwrap();
+        assert_eq!(ub.len(), 512);
+        let sb = Superblock::parse(hdf5, 0).unwrap();
         assert_eq!(sb.version, 0);
         assert_eq!(sb.root_group_address, 96);
     }
diff --git a/crates/clawhdf5-io/src/async_read.rs b/crates/clawhdf5-io/src/async_read.rs
index 94afff0..d9e2ffe 100644
--- a/crates/clawhdf5-io/src/async_read.rs
+++ b/crates/clawhdf5-io/src/async_read.rs
@@ -268,28 +268,26 @@ impl AsyncHDF5File {
     ///
     /// Reads the entire file into memory, then parses the superblock.
     pub async fn open<R: AsyncHDF5Read>(reader: &R) -> Result<Self, AsyncHDF5Error> {
-        let data = reader.read_all().await?;
-        let sig_offset = find_signature(&data)?;
-        let superblock = Superblock::parse(&data, sig_offset)?;
-        Ok(Self { data, superblock })
+        Self::from_bytes(reader.read_all().await?)
     }
 
     /// Open an HDF5 file asynchronously from a file path.
     pub async fn open_path<P: AsRef<Path>>(path: P) -> Result<Self, AsyncHDF5Error> {
-        let data = tokio::fs::read(path).await?;
-        let sig_offset = find_signature(&data)?;
-        let superblock = Superblock::parse(&data, sig_offset)?;
-        Ok(Self { data, superblock })
+        Self::from_bytes(tokio::fs::read(path).await?)
     }
 
     /// Open an HDF5 file from bytes already in memory.
-    pub fn from_bytes(data: Vec<u8>) -> Result<Self, AsyncHDF5Error> {
-        let sig_offset = find_signature(&data)?;
-        let superblock = Superblock::parse(&data, sig_offset)?;
+    pub fn from_bytes(mut data: Vec<u8>) -> Result<Self, AsyncHDF5Error> {
+        // HDF5 addresses are relative to the superblock: drop any user block
+        // so they index `data` directly.
+        let user_block = find_signature(&data)?;
+        data.drain(..user_block);
+        let superblock = Superblock::parse(&data, 0)?;
         Ok(Self { data, superblock })
     }
 
-    /// Access the raw file bytes.
+    /// Access the file bytes from the superblock on (any user block is
+    /// dropped on open).
     pub fn as_bytes(&self) -> &[u8] {
         &self.data
     }
diff --git a/crates/clawhdf5-io/src/mpi_vol.rs b/crates/clawhdf5-io/src/mpi_vol.rs
index a1ddf72..7dac39e 100644
--- a/crates/clawhdf5-io/src/mpi_vol.rs
+++ b/crates/clawhdf5-io/src/mpi_vol.rs
@@ -180,7 +180,7 @@ fn mpi_collective_read(vol: &MpiVol, location: &str, path: &str) -> Result<Vec<u
     use clawhdf5_format::{
         data_layout::DataLayout, data_read::read_raw_data_full, dataspace::Dataspace,
         datatype::Datatype, filter_pipeline::FilterPipeline, group_v2::resolve_path_any,
-        message_type::MessageType, object_header::ObjectHeader, signature::find_signature,
+        message_type::MessageType, object_header::ObjectHeader, signature::split_user_block,
         superblock::Superblock,
     };
     use mpi::traits::*;
@@ -192,9 +192,10 @@ fn mpi_collective_read(vol: &MpiVol, location: &str, path: &str) -> Result<Vec<u
     let mut len_buf = [0usize; 1];
 
     if rank == 0 {
-        let bytes = std::fs::read(location).map_err(VolError::Io)?;
-        let sig = find_signature(&bytes).map_err(|e| VolError::DataError(e.to_string()))?;
-        let sb = Superblock::parse(&bytes, sig).map_err(|e| VolError::DataError(e.to_string()))?;
+        let file = std::fs::read(location).map_err(VolError::Io)?;
+        // Addresses are relative to the superblock: skip any user block.
+        let (_, bytes) = split_user_block(&file).map_err(|e| VolError::DataError(e.to_string()))?;
+        let sb = Superblock::parse(bytes, 0).map_err(|e| VolError::DataError(e.to_string()))?;
         let addr = resolve_path_any(&bytes, &sb, path)
             .map_err(|e| VolError::NotFound(format!("{path}: {e}")))?;
         let oh = ObjectHeader::parse(&bytes, addr as usize, sb.offset_size, sb.length_size)
diff --git a/crates/clawhdf5-io/src/vol.rs b/crates/clawhdf5-io/src/vol.rs
index 4036391..47e1388 100644
--- a/crates/clawhdf5-io/src/vol.rs
+++ b/crates/clawhdf5-io/src/vol.rs
@@ -283,12 +283,13 @@ impl VirtualObjectLayer for NativeVol {
         use clawhdf5_format::{
             data_layout::DataLayout, data_read::read_raw_data_full, dataspace::Dataspace,
             datatype::Datatype, filter_pipeline::FilterPipeline, group_v2::resolve_path_any,
-            message_type::MessageType, object_header::ObjectHeader, signature::find_signature,
+            message_type::MessageType, object_header::ObjectHeader, signature::split_user_block,
             superblock::Superblock,
         };
 
-        let sig = find_signature(data).map_err(|e| VolError::DataError(e.to_string()))?;
-        let sb = Superblock::parse(data, sig).map_err(|e| VolError::DataError(e.to_string()))?;
+        // Addresses are relative to the superblock: skip any user block.
+        let (_, data) = split_user_block(data).map_err(|e| VolError::DataError(e.to_string()))?;
+        let sb = Superblock::parse(data, 0).map_err(|e| VolError::DataError(e.to_string()))?;
         let addr = resolve_path_any(data, &sb, path)
             .map_err(|e| VolError::NotFound(format!("{path}: {e}")))?;
 
diff --git a/crates/clawhdf5/src/lazy.rs b/crates/clawhdf5/src/lazy.rs
index 4a7421e..9099246 100644
--- a/crates/clawhdf5/src/lazy.rs
+++ b/crates/clawhdf5/src/lazy.rs
@@ -42,6 +42,9 @@ use crate::types::{AttrValue, DType, attrs_to_map, classify_datatype};
 /// `MemoryReader`, etc.
 pub struct LazyFile<R: HDF5Read> {
     reader: R,
+    /// Offset of the superblock in the file (the user-block size); every
+    /// HDF5 address is relative to it.
+    base: usize,
     superblock: Superblock,
     root_header: ObjectHeader,
     /// Cache of parsed object headers, keyed by address.
@@ -73,9 +76,9 @@ impl<R: HDF5Read> LazyFile<R> {
     ///
     /// Parses only the superblock and root group object header.
     pub fn open(reader: R) -> Result<Self, Error> {
-        let data = reader.as_bytes();
-        let sig_offset = signature::find_signature(data)?;
-        let superblock = Superblock::parse(data, sig_offset)?;
+        let (user_block, data) = signature::split_user_block(reader.as_bytes())?;
+        let base = user_block.len();
+        let superblock = Superblock::parse(data, 0)?;
         let root_header = ObjectHeader::parse(
             data,
             superblock.root_group_address as usize,
@@ -84,15 +87,26 @@ impl<R: HDF5Read> LazyFile<R> {
         )?;
         Ok(Self {
             reader,
+            base,
             superblock,
             root_header,
             header_cache: RefCell::new(HashMap::new()),
         })
     }
 
-    /// Returns the raw file bytes.
+    /// Returns the file's bytes from the superblock on (after any user
+    /// block), which is the space every HDF5 address in the file indexes.
     pub fn as_bytes(&self) -> &[u8] {
-        self.reader.as_bytes()
+        self.hdf5_bytes()
+    }
+
+    /// Size of the user block before the superblock (0 for most files).
+    pub fn user_block_size(&self) -> u64 {
+        self.base as u64
+    }
+
+    fn hdf5_bytes(&self) -> &[u8] {
+        &self.reader.as_bytes()[self.base..]
     }
 
     /// Returns a reference to the parsed superblock.
@@ -110,7 +124,7 @@ impl<R: HDF5Read> LazyFile<R> {
 
     /// Resolve a path and return a `LazyDataset` handle.
     pub fn dataset(&self, path: &str) -> Result<LazyDataset<'_, R>, Error> {
-        let data = self.reader.as_bytes();
+        let data = self.hdf5_bytes();
         let addr = group_v2::resolve_path_any(data, &self.superblock, path)?;
         let hdr = self.get_or_parse_header(addr)?;
         if !has_message(&hdr, MessageType::DataLayout) {
@@ -124,7 +138,7 @@ impl<R: HDF5Read> LazyFile<R> {
 
     /// Resolve a path and return a `LazyGroup` handle.
     pub fn group(&self, path: &str) -> Result<LazyGroup<'_, R>, Error> {
-        let data = self.reader.as_bytes();
+        let data = self.hdf5_bytes();
         let addr = group_v2::resolve_path_any(data, &self.superblock, path)?;
         Ok(LazyGroup {
             file: self,
@@ -163,7 +177,7 @@ impl<R: HDF5Read> LazyFile<R> {
         }
 
         // Parse and cache
-        let data = self.reader.as_bytes();
+        let data = self.hdf5_bytes();
         let hdr = ObjectHeader::parse(
             data,
             address as usize,
@@ -187,7 +201,7 @@ impl<R: HDF5Read> LazyFile<R> {
 impl<R: HDF5Read> std::fmt::Debug for LazyFile<R> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("LazyFile")
-            .field("size", &self.reader.as_bytes().len())
+            .field("size", &self.hdf5_bytes().len())
             .field("superblock_version", &self.superblock.version)
             .field("cached_headers", &self.header_cache.borrow().len())
             .finish()
@@ -234,7 +248,7 @@ impl<'f, R: HDF5Read> LazyGroup<'f, R> {
     /// Read all attributes of this group.
     pub fn attrs(&self) -> Result<HashMap<String, AttrValue>, Error> {
         let hdr = self.file.get_or_parse_header(self.address)?;
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let attr_msgs =
             extract_attributes_full(data, &hdr, self.file.offset_size(), self.file.length_size())?;
         Ok(attrs_to_map(
@@ -277,7 +291,7 @@ impl<'f, R: HDF5Read> LazyGroup<'f, R> {
 
     fn children(&self) -> Result<Vec<GroupEntry>, Error> {
         let hdr = self.file.get_or_parse_header(self.address)?;
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let os = self.file.offset_size();
         let ls = self.file.length_size();
         resolve_group_entries(data, &hdr, os, ls).map_err(Error::Format)
@@ -360,7 +374,7 @@ impl<'f, R: HDF5Read> LazyDataset<'f, R> {
         let dl = self.data_layout()?;
         let ds = self.dataspace()?;
         let dt = self.datatype()?;
-        let slice = data_read::read_raw_data_zerocopy(self.file.reader.as_bytes(), &dl, &ds, &dt)?;
+        let slice = data_read::read_raw_data_zerocopy(self.file.hdf5_bytes(), &dl, &ds, &dt)?;
         Ok(slice)
     }
 
@@ -401,7 +415,7 @@ impl<'f, R: HDF5Read> LazyDataset<'f, R> {
 
     /// Read all attributes of this dataset.
     pub fn attrs(&self) -> Result<HashMap<String, AttrValue>, Error> {
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let attr_msgs = extract_attributes_full(
             data,
             &self.header,
@@ -479,7 +493,7 @@ impl<'f, R: HDF5Read> LazyDataset<'f, R> {
         let ds = self.dataspace()?;
         let dl = self.data_layout()?;
         let pipeline = self.filter_pipeline()?;
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         // Unallocated storage reads as the dataset's fill value.
         clawhdf5_format::fill_value::read_full_with_fill(
             &self.header.messages,
diff --git a/crates/clawhdf5/src/mmap_file.rs b/crates/clawhdf5/src/mmap_file.rs
index 9119cdf..b7251ac 100644
--- a/crates/clawhdf5/src/mmap_file.rs
+++ b/crates/clawhdf5/src/mmap_file.rs
@@ -34,6 +34,9 @@ use crate::types::{AttrValue, DType, attrs_to_map, classify_datatype};
 /// `&[u8]` slice via [`MmapDataset::read_raw_slice`].
 pub struct MmapFile {
     reader: MmapReader,
+    /// Offset of the superblock in the mapped file (the user-block size);
+    /// every HDF5 address is relative to it.
+    base: usize,
     superblock: Superblock,
 }
 
@@ -41,10 +44,25 @@ impl MmapFile {
     /// Open an HDF5 file using memory-mapped I/O.
     pub fn open<P: AsRef<std::path::Path>>(path: P) -> Result<Self, Error> {
         let reader = MmapReader::open(path).map_err(Error::Io)?;
-        let data = reader.as_bytes();
-        let sig_offset = signature::find_signature(data)?;
-        let superblock = Superblock::parse(data, sig_offset)?;
-        Ok(Self { reader, superblock })
+        let (user_block, data) = signature::split_user_block(reader.as_bytes())?;
+        let base = user_block.len();
+        let superblock = Superblock::parse(data, 0)?;
+        Ok(Self {
+            reader,
+            base,
+            superblock,
+        })
+    }
+
+    /// The file's bytes from the superblock on — the space HDF5 addresses
+    /// index into.
+    fn hdf5_bytes(&self) -> &[u8] {
+        &self.reader.as_bytes()[self.base..]
+    }
+
+    /// Size of the user block before the superblock (0 for most files).
+    pub fn user_block_size(&self) -> u64 {
+        self.base as u64
     }
 
     /// Returns a handle to the root group.
@@ -57,7 +75,7 @@ impl MmapFile {
 
     /// Resolve a path and return a `MmapDataset` handle.
     pub fn dataset(&self, path: &str) -> Result<MmapDataset<'_>, Error> {
-        let data = self.reader.as_bytes();
+        let data = self.hdf5_bytes();
         let addr = group_v2::resolve_path_any(data, &self.superblock, path)?;
         let hdr = self.parse_header(addr)?;
         if !has_message(&hdr, MessageType::DataLayout) {
@@ -71,7 +89,7 @@ impl MmapFile {
 
     /// Resolve a path and return a `MmapGroup` handle.
     pub fn group(&self, path: &str) -> Result<MmapGroup<'_>, Error> {
-        let data = self.reader.as_bytes();
+        let data = self.hdf5_bytes();
         let addr = group_v2::resolve_path_any(data, &self.superblock, path)?;
         Ok(MmapGroup {
             file: self,
@@ -79,9 +97,11 @@ impl MmapFile {
         })
     }
 
-    /// Returns the raw file bytes (zero-copy from mmap).
+    /// Returns the file's bytes from the superblock on (after any user
+    /// block), zero-copy from the mmap. Every HDF5 address in the file
+    /// indexes this slice.
     pub fn as_bytes(&self) -> &[u8] {
-        self.reader.as_bytes()
+        self.hdf5_bytes()
     }
 
     /// Returns a reference to the parsed superblock.
@@ -91,7 +111,7 @@ impl MmapFile {
 
     fn parse_header(&self, address: u64) -> Result<ObjectHeader, FormatError> {
         ObjectHeader::parse(
-            self.reader.as_bytes(),
+            self.hdf5_bytes(),
             address as usize,
             self.superblock.offset_size,
             self.superblock.length_size,
@@ -155,7 +175,7 @@ impl<'f> MmapGroup<'f> {
 
     /// Read all attributes of this group.
     pub fn attrs(&self) -> Result<HashMap<String, AttrValue>, Error> {
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let hdr = self.file.parse_header(self.address)?;
         let attr_msgs =
             extract_attributes_full(data, &hdr, self.file.offset_size(), self.file.length_size())?;
@@ -198,7 +218,7 @@ impl<'f> MmapGroup<'f> {
     }
 
     fn children(&self) -> Result<Vec<GroupEntry>, Error> {
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let hdr = self.file.parse_header(self.address)?;
         let os = self.file.offset_size();
         let ls = self.file.length_size();
@@ -326,7 +346,7 @@ impl<'f> MmapDataset<'f> {
                         actual: sz,
                     }));
                 }
-                let data = self.file.reader.as_bytes();
+                let data = self.file.hdf5_bytes();
                 let a = addr as usize;
                 if a + sz > data.len() {
                     return Err(Error::Format(FormatError::UnexpectedEof {
@@ -342,7 +362,7 @@ impl<'f> MmapDataset<'f> {
 
     /// Read all attributes of this dataset.
     pub fn attrs(&self) -> Result<HashMap<String, AttrValue>, Error> {
-        let data = self.file.reader.as_bytes();
+        let data = self.file.hdf5_bytes();
         let attr_msgs = extract_attributes_full(
             data,
             &self.header,
@@ -423,7 +443,7 @@ impl<'f> MmapDataset<'f> {
         // Unallocated storage reads as the dataset's fill value.
         clawhdf5_format::fill_value::read_full_with_fill(
             &self.header.messages,
-            self.file.reader.as_bytes(),
+            self.file.hdf5_bytes(),
             &dl,
             &ds,
             dt.type_size() as usize,
@@ -431,7 +451,7 @@ impl<'f> MmapDataset<'f> {
             self.file.length_size(),
             || {
                 Ok(data_read::read_raw_data_full(
-                    self.file.reader.as_bytes(),
+                    self.file.hdf5_bytes(),
                     &dl,
                     &ds,
                     &dt,
diff --git a/crates/clawhdf5/src/reader.rs b/crates/clawhdf5/src/reader.rs
index bb6a8fd..b3d7338 100644
--- a/crates/clawhdf5/src/reader.rs
+++ b/crates/clawhdf5/src/reader.rs
@@ -31,20 +31,43 @@ use crate::types::{AttrValue, DType, attrs_to_map, classify_datatype};
 // ---------------------------------------------------------------------------
 
 /// Internal storage: either an owned `Vec<u8>` or a memory-mapped region.
-enum FileData {
+enum Backing {
     Owned(Vec<u8>),
     #[cfg(feature = "mmap")]
     Mmap(clawhdf5_io::MmapReader),
 }
 
-impl FileData {
-    fn as_bytes(&self) -> &[u8] {
+impl Backing {
+    fn whole_file(&self) -> &[u8] {
         match self {
-            FileData::Owned(v) => v,
+            Backing::Owned(v) => v,
             #[cfg(feature = "mmap")]
-            FileData::Mmap(r) => r.as_bytes(),
+            Backing::Mmap(r) => r.as_bytes(),
         }
     }
+}
+
+/// The file's bytes, viewed from the superblock on. A file may start with a
+/// user block (the superblock at 512, 1024, …); every HDF5 address is
+/// relative to the superblock, so all parsing goes through [`Self::as_bytes`].
+struct FileData {
+    backing: Backing,
+    /// Offset of the superblock in the file (the user-block size).
+    base: usize,
+}
+
+impl FileData {
+    /// Locate the superblock and parse it.
+    fn new(backing: Backing) -> Result<(Self, Superblock), Error> {
+        let (user_block, hdf5) = signature::split_user_block(backing.whole_file())?;
+        let base = user_block.len();
+        let superblock = Superblock::parse(hdf5, 0)?;
+        Ok((Self { backing, base }, superblock))
+    }
+
+    fn as_bytes(&self) -> &[u8] {
+        &self.backing.whole_file()[self.base..]
+    }
 
     fn len(&self) -> usize {
         self.as_bytes().len()
@@ -81,11 +104,9 @@ impl File {
         #[cfg(feature = "mmap")]
         {
             let reader = clawhdf5_io::MmapReader::open(path).map_err(Error::Io)?;
-            let data_ref = reader.as_bytes();
-            let sig_offset = signature::find_signature(data_ref)?;
-            let superblock = Superblock::parse(data_ref, sig_offset)?;
+            let (data, superblock) = FileData::new(Backing::Mmap(reader))?;
             Ok(Self {
-                data: FileData::Mmap(reader),
+                data,
                 superblock,
                 chunk_cache: ChunkCache::new(),
                 base_dir,
@@ -116,10 +137,9 @@ impl File {
     /// In-memory files have no directory, so external Virtual Dataset sources
     /// cannot be resolved automatically (same-file VDS still works).
     pub fn from_bytes(data: Vec<u8>) -> Result<Self, Error> {
-        let sig_offset = signature::find_signature(&data)?;
-        let superblock = Superblock::parse(&data, sig_offset)?;
+        let (data, superblock) = FileData::new(Backing::Owned(data))?;
         Ok(Self {
-            data: FileData::Owned(data),
+            data,
             superblock,
             chunk_cache: ChunkCache::new(),
             base_dir: None,
@@ -209,11 +229,19 @@ impl File {
         Ok(results.into_iter().map(|(_, data)| data).collect())
     }
 
-    /// Returns the raw file bytes.
+    /// Returns the file's bytes from the superblock on (after any user
+    /// block). Every HDF5 address in the file indexes this slice, so it is
+    /// what the `clawhdf5_format` parsers expect as `file_data`.
     pub fn as_bytes(&self) -> &[u8] {
         self.data.as_bytes()
     }
 
+    /// Size of the user block before the superblock (0 for most files).
+    /// Matches h5py's `File.userblock_size`.
+    pub fn user_block_size(&self) -> u64 {
+        self.data.base as u64
+    }
+
     /// Returns a reference to the parsed superblock.
     pub fn superblock(&self) -> &Superblock {
         &self.superblock
@@ -221,10 +249,10 @@ impl File {
 
     /// Returns `true` when the file is backed by memory-mapped I/O.
     pub fn is_mmap(&self) -> bool {
-        match &self.data {
-            FileData::Owned(_) => false,
+        match &self.data.backing {
+            Backing::Owned(_) => false,
             #[cfg(feature = "mmap")]
-            FileData::Mmap(_) => true,
+            Backing::Mmap(_) => true,
         }
     }
 
diff --git a/crates/clawhdf5/tests/userblock_interop.rs b/crates/clawhdf5/tests/userblock_interop.rs
new file mode 100644
index 0000000..c9130b9
--- /dev/null
+++ b/crates/clawhdf5/tests/userblock_interop.rs
@@ -0,0 +1,314 @@
+//! Files that start with a user block (`h5py.File(..., userblock_size=N)`,
+//! `h5jam`): the superblock sits at 512, 1024, ... and every address in the
+//! file is relative to it. Each reader (buffered, mmap, `MmapFile`,
+//! `LazyFile`) must apply that base, and read the same values h5py does.
+//!
+//! h5py writes the files; skipped when python3 with h5py is unavailable,
+//! unless `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+use std::collections::HashMap;
+use std::path::Path;
+use std::process::Command;
+
+use clawhdf5::{AttrValue, File, LazyFile, MmapFile};
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+fn python_available() -> bool {
+    Command::new(python())
+        .args(["-c", "import h5py, numpy"])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+macro_rules! skip_if_no_python {
+    () => {
+        if !python_available() {
+            assert!(
+                !interop_required(),
+                "CLAWHDF5_REQUIRE_INTEROP=1 but python3 with h5py is not available"
+            );
+            eprintln!("SKIP: python3 with h5py not available");
+            return;
+        }
+    };
+}
+
+/// Run `script` and return its stdout as `key -> values` (one
+/// `key v1 v2 ...` line per key).
+fn run_python(script: &str) -> HashMap<String, Vec<String>> {
+    let output = Command::new(python())
+        .args(["-c", script])
+        .output()
+        .expect("failed to run python");
+    assert!(
+        output.status.success(),
+        "python failed:\n{}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    String::from_utf8_lossy(&output.stdout)
+        .lines()
+        .filter_map(|line| {
+            let mut words = line.split_whitespace().map(str::to_string);
+            Some((words.next()?, words.collect()))
+        })
+        .collect()
+}
+
+fn parse<T: std::str::FromStr>(values: &[String]) -> Vec<T>
+where
+    T::Err: std::fmt::Debug,
+{
+    values.iter().map(|v| v.parse().unwrap()).collect()
+}
+
+/// Write a file with a user block of `userblock` bytes holding contiguous,
+/// chunked (deflate), compact and committed-type datasets, nested groups,
+/// and attributes (compact and, under `latest`, dense). Prints what h5py
+/// reads back.
+fn write_file(path: &Path, userblock: u32, libver: &str) -> HashMap<String, Vec<String>> {
+    let script = format!(
+        r#"
+import h5py, numpy as np
+path = "{path}"
+with h5py.File(path, "w", userblock_size={userblock}, libver={libver}) as f:
+    f.attrs["title"] = "user block"
+    f.attrs["answer"] = np.int64(42)
+    f.create_dataset("contig", data=np.arange(12, dtype="<f8") * 0.5)
+    f.create_dataset("chunked", data=np.arange(1000, dtype="<i4") * 3 - 7,
+                     chunks=(128,), compression="gzip")
+    dcpl = h5py.h5p.create(h5py.h5p.DATASET_CREATE)
+    dcpl.set_layout(h5py.h5d.COMPACT)
+    space = h5py.h5s.create_simple((5,))
+    dsid = h5py.h5d.create(f.id, b"compact", h5py.h5t.STD_I64LE, space, dcpl=dcpl)
+    dsid.write(h5py.h5s.ALL, h5py.h5s.ALL, np.array([5, -4, 3, -2, 1], "<i8"))
+    f["named_type"] = np.dtype("<f4")
+    f.create_dataset("committed", data=np.array([1.25, -2.5], "<f4"),
+                     dtype=f["named_type"])
+    g = f.create_group("a/b")
+    g.create_dataset("deep", data=np.array([7, 8, 9], "<i8"))
+    g.attrs["scale"] = 2.5
+    d = f["contig"]
+    d.attrs["units"] = "m"
+    # Enough attributes that `latest` stores them densely (fractal heap).
+    for i in range(12):
+        f["a"].attrs["k%02d" % i] = np.int64(i * i)
+    # And enough links for a dense (fractal-heap) group under `latest`.
+    many = f.create_group("many")
+    for i in range(20):
+        many.create_dataset("d%02d" % i, data=np.array([i], "<i4"))
+with h5py.File(path, "r") as f:
+    print("userblock", f.userblock_size)
+    print("contig", *f["contig"][()])
+    print("chunked", *f["chunked"][()])
+    print("compact", *f["compact"][()])
+    print("committed", *f["committed"][()])
+    print("deep", *f["a/b/deep"][()])
+    print("many", *[int(f["many/d%02d" % i][0]) for i in range(20)])
+    print("k", *[int(f["a"].attrs["k%02d" % i]) for i in range(12)])
+"#,
+        path = path.display(),
+        libver = if libver == "default" {
+            "None".to_string()
+        } else {
+            format!("{libver:?}")
+        },
+    );
+    run_python(&script)
+}
+
+/// Attribute value rendered for comparison (`AttrValue` has no `PartialEq`).
+fn attr(map: &HashMap<String, AttrValue>, key: &str) -> String {
+    match map.get(key) {
+        Some(AttrValue::I64(v)) => format!("i64 {v}"),
+        Some(AttrValue::F64(v)) => format!("f64 {v}"),
+        Some(AttrValue::String(v)) => format!("str {v}"),
+        other => format!("{other:?}"),
+    }
+}
+
+fn i64s(v: &[String]) -> Vec<i64> {
+    parse(v)
+}
+
+/// Everything read through the `File` API must match h5py.
+fn check_file(file: &File, expected: &HashMap<String, Vec<String>>, label: &str) {
+    let ub: u64 = expected["userblock"][0].parse().unwrap();
+    assert_eq!(file.user_block_size(), ub, "{label}: user block size");
+    assert_eq!(
+        file.dataset("contig").unwrap().read_f64().unwrap(),
+        parse::<f64>(&expected["contig"]),
+        "{label}: contiguous"
+    );
+    assert_eq!(
+        file.dataset("chunked")
+            .unwrap()
+            .read_i32()
+            .unwrap()
+            .iter()
+            .map(|&v| v as i64)
+            .collect::<Vec<_>>(),
+        i64s(&expected["chunked"]),
+        "{label}: chunked"
+    );
+    assert_eq!(
+        file.dataset("compact").unwrap().read_i64().unwrap(),
+        i64s(&expected["compact"]),
+        "{label}: compact"
+    );
+    assert_eq!(
+        file.dataset("committed").unwrap().read_f32().unwrap(),
+        parse::<f32>(&expected["committed"]),
+        "{label}: committed datatype"
+    );
+    assert_eq!(
+        file.dataset("a/b/deep").unwrap().read_i64().unwrap(),
+        i64s(&expected["deep"]),
+        "{label}: nested group"
+    );
+    let many: Vec<i64> = (0..20)
+        .map(|i| {
+            file.dataset(&format!("many/d{i:02}"))
+                .unwrap()
+                .read_i32()
+                .unwrap()[0] as i64
+        })
+        .collect();
+    assert_eq!(many, i64s(&expected["many"]), "{label}: many links");
+
+    let root = file.root().attrs().unwrap();
+    assert_eq!(attr(&root, "title"), "str user block", "{label}");
+    assert_eq!(attr(&root, "answer"), "i64 42", "{label}");
+    let a = file.group("a").unwrap().attrs().unwrap();
+    let k: Vec<i64> = (0..12)
+        .map(|i| match &a[&format!("k{i:02}")] {
+            AttrValue::I64(v) => *v,
+            _ => panic!("{label}: k{i:02} is not an i64"),
+        })
+        .collect();
+    assert_eq!(k, i64s(&expected["k"]), "{label}: attributes");
+    assert_eq!(
+        attr(&file.group("a/b").unwrap().attrs().unwrap(), "scale"),
+        "f64 2.5",
+        "{label}"
+    );
+    assert_eq!(
+        attr(&file.dataset("contig").unwrap().attrs().unwrap(), "units"),
+        "str m",
+        "{label}"
+    );
+}
+
+fn check_all_readers(path: &Path, expected: &HashMap<String, Vec<String>>, label: &str) {
+    check_file(
+        &File::open(path).unwrap(),
+        expected,
+        &format!("{label} File::open"),
+    );
+    check_file(
+        &File::open_buffered(path).unwrap(),
+        expected,
+        &format!("{label} File::open_buffered"),
+    );
+    check_file(
+        &File::from_bytes(std::fs::read(path).unwrap()).unwrap(),
+        expected,
+        &format!("{label} File::from_bytes"),
+    );
+
+    let ub: u64 = expected["userblock"][0].parse().unwrap();
+
+    let mm = MmapFile::open(path).unwrap();
+    assert_eq!(mm.user_block_size(), ub, "{label} MmapFile");
+    assert_eq!(
+        mm.dataset("contig").unwrap().read_f64().unwrap(),
+        parse::<f64>(&expected["contig"]),
+        "{label} MmapFile contiguous"
+    );
+    assert_eq!(
+        mm.dataset("compact").unwrap().read_i64().unwrap(),
+        i64s(&expected["compact"]),
+        "{label} MmapFile compact"
+    );
+    assert_eq!(
+        mm.dataset("committed").unwrap().read_f32().unwrap(),
+        parse::<f32>(&expected["committed"]),
+        "{label} MmapFile committed"
+    );
+    assert_eq!(
+        mm.dataset("a/b/deep").unwrap().read_i64().unwrap(),
+        i64s(&expected["deep"]),
+        "{label} MmapFile nested"
+    );
+    assert_eq!(
+        attr(&mm.root().attrs().unwrap(), "answer"),
+        "i64 42",
+        "{label} MmapFile attrs"
+    );
+
+    let lazy = LazyFile::open_mmap(path).unwrap();
+    assert_eq!(lazy.user_block_size(), ub, "{label} LazyFile");
+    assert_eq!(
+        lazy.dataset("contig").unwrap().read_f64().unwrap(),
+        parse::<f64>(&expected["contig"]),
+        "{label} LazyFile contiguous"
+    );
+    assert_eq!(
+        lazy.dataset("chunked")
+            .unwrap()
+            .read_i32()
+            .unwrap()
+            .iter()
+            .map(|&v| v as i64)
+            .collect::<Vec<_>>(),
+        i64s(&expected["chunked"]),
+        "{label} LazyFile chunked"
+    );
+    assert_eq!(
+        lazy.dataset("committed").unwrap().read_f32().unwrap(),
+        parse::<f32>(&expected["committed"]),
+        "{label} LazyFile committed"
+    );
+    assert_eq!(
+        lazy.dataset("a/b/deep").unwrap().read_i64().unwrap(),
+        i64s(&expected["deep"]),
+        "{label} LazyFile nested"
+    );
+    assert_eq!(
+        attr(&lazy.root().attrs().unwrap(), "answer"),
+        "i64 42",
+        "{label} LazyFile attrs"
+    );
+}
+
+#[test]
+fn user_block_files_read_like_h5py() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    for userblock in [512u32, 4096] {
+        for libver in ["default", "latest"] {
+            let label = format!("userblock={userblock} libver={libver}");
+            let path = dir.path().join(format!("ub_{userblock}_{libver}.h5"));
+            let expected = write_file(&path, userblock, libver);
+            assert_eq!(expected["userblock"], [userblock.to_string()], "{label}");
+            check_all_readers(&path, &expected, &label);
+        }
+    }
+}
+
+#[test]
+fn file_without_user_block_reports_zero() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("no_ub.h5");
+    let expected = write_file(&path, 0, "default");
+    assert_eq!(expected["userblock"], ["0"]);
+    check_all_readers(&path, &expected, "userblock=0");
+}
diff --git a/docs/known-issues.md b/docs/known-issues.md
index 0783e91..6f84c6a 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -73,6 +73,9 @@ the VDS item, which is marked.
   - `%b` printf-style source names are not expanded.
   - Hyperslab selection versions 1 and 2 are refused.
 - **Files with a user block:** the base address is not applied.
+  **Fixed 2026-09-25:** every reader views the file from the superblock on
+  (`twithub.h5`, `twithub513.h5`, `h5clear_fsm_persist_user_*.h5`; the
+  `twithub` files still stop at the user-defined link type below).
 - **Old-style shared messages (version 1)** read the wrong address.
   **Fixed 2026-09-25:** the address follows a length-sized heap offset
   (`tcompound.h5`, `tcompound2.h5`; their datasets now stop at the layout