diff --git a/newsfragments/651.feature b/newsfragments/651.feature
new file mode 100644
index 000000000..2f790c707
--- /dev/null
+++ b/newsfragments/651.feature
@@ -0,0 +1 @@
+Add support for Eiger 9M on ESRF ID23-2, which has an undeclared vertical goniometer.
diff --git a/src/dxtbx/format/FormatNXmxEigerFilewriterESRFID232.py b/src/dxtbx/format/FormatNXmxEigerFilewriterESRFID232.py
new file mode 100644
index 000000000..58d2fcb60
--- /dev/null
+++ b/src/dxtbx/format/FormatNXmxEigerFilewriterESRFID232.py
@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+import re
+
+import h5py
+
+from dxtbx.format.FormatNXmxEigerFilewriter import FormatNXmxEigerFilewriter
+
+DATA_FILE_RE = re.compile(r"data_\d{6}")
+
+
+class FormatNXmxEigerFilewriterESRFID232(FormatNXmxEigerFilewriter):
+    _cached_file_handle = None
+
+    @staticmethod
+    def understand(image_file):
+        with h5py.File(image_file) as handle:
+            if "/entry/instrument/detector/detector_number" in handle:
+                if (
+                    handle["/entry/instrument/detector/detector_number"][()]
+                    == b"E-18-0133"
+                ):
+                    return True
+        return False
+
+    def __init__(self, image_file, **kwargs):
+        """Initialise the image structure from the given file."""
+        super().__init__(image_file, **kwargs)
+
+    def _start(self):
+        super()._start()
+
+    def _goniometer(self):
+        return self._goniometer_factory.known_axis((0, 1, 0))