diff --git a/RELEASE.md b/RELEASE.md
index 29beb5ae..25ba39f3 100644
--- a/RELEASE.md
+++ b/RELEASE.md
@@ -74,6 +74,9 @@
 - ``TimingMode::Auto`` changed to ``TimingMode::AUTO_TIMING``, ``TimingMode::Trigger`` changed to ``TimingMode::TRIGGER_EXPOSURE``
 
 ### Bugfixes:
+- Fixed ``CtbRawFile.read_frame(index)`` and reads after ``seek(index)`` at
+  subfile boundaries skipping a subfile, returning the wrong frame or raising
+  ``Subfile index out of range``.
 - ``Pedestal`` reports mismatched frame shapes with exceptions in all push
   overloads, including Debug builds, instead of aborting on assertions.
 - Python ``Pedestal`` constructors reject negative dimensions and sample
diff --git a/docs/src/python/file/pyCtbRawFile.rst b/docs/src/python/file/pyCtbRawFile.rst
index 75d83550..f0676e6d 100644
--- a/docs/src/python/file/pyCtbRawFile.rst
+++ b/docs/src/python/file/pyCtbRawFile.rst
@@ -6,6 +6,10 @@ Read analog, digital and transceiver samples from a raw file containing
 data from the Chip Test Board. Uses :mod:`aare.transform` to decode the 
 data into a format that the user can work with. 
 
+``read_frame(index)`` reads the frame at the zero-based index across the raw
+subfiles and advances the position to ``index + 1``. A subsequent
+``read_frame()`` reads the following frame, including across subfile boundaries.
+
 .. code:: python
 
     import aare
@@ -22,4 +26,4 @@ data into a format that the user can work with.
     :members:
     :undoc-members:
     :show-inheritance:
-    :inherited-members:
\ No newline at end of file
+    :inherited-members:
diff --git a/python/tests/test_CtbRawFile.py b/python/tests/test_CtbRawFile.py
new file mode 100644
index 00000000..6485b076
--- /dev/null
+++ b/python/tests/test_CtbRawFile.py
@@ -0,0 +1,58 @@
+
+
+import pytest
+import numpy as np
+from aare import CtbRawFile
+
+
+@pytest.mark.withdata
+def test_basic_properites(test_data_path):
+    f = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+    assert f.total_frames == 47
+    assert f.image_size_in_bytes == 640
+    assert f.master.analog_samples == 10
+    assert f.master.digital_samples is None
+    assert f.master.transceiver_samples is None
+
+
+@pytest.mark.withdata
+def test_seek_and_read(test_data_path):
+    f = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+    f.seek(0)
+    header, raw_data = f.read_frame()
+    assert header['frameNumber'] == 48 #we know that the first frame in this file is 48
+
+    #Seek a multiple of frames in file
+    f.seek(9)
+    header, raw_data = f.read_frame()
+    assert header['frameNumber'] == 57 
+
+@pytest.mark.withdata
+def test_read_all_frames_direct_and_iterate(test_data_path):
+    """
+    We want to make sure that iterating and reading give the same result.
+    """
+    f0 = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+    f1 = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+
+    for h0, d0 in f0:
+        h1, d1 = f1.read_frame()
+        assert h0 == h1
+        assert np.all(d0 == d1)
+
+@pytest.mark.withdata
+def test_read_all_frames_with_index_and_iterate(test_data_path):
+    """
+    We want to make sure that iterating and reading give the same result.
+    """
+    f0 = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+    f1 = CtbRawFile(test_data_path/'raw/ctb/run_master_0.json')
+
+    frames_read = 0
+    for i,(h0, d0) in enumerate(f0):
+        h1, d1 = f1.read_frame(i)
+        assert h0 == h1
+        assert np.all(d0 == d1)
+        frames_read += 1
+
+    assert frames_read == f0.total_frames
diff --git a/src/CtbRawFile.cpp b/src/CtbRawFile.cpp
index 260ab668..e4d0b958 100644
--- a/src/CtbRawFile.cpp
+++ b/src/CtbRawFile.cpp
@@ -24,9 +24,9 @@ void CtbRawFile::read_into(std::byte *image_buf, DetectorHeader *header) {
         throw std::runtime_error(LOCATION + " End of file reached");
     }
 
-    if (m_current_frame != 0 &&
-        m_current_frame % m_master.max_frames_per_file() == 0) {
-        open_data_file(m_current_subfile + 1);
+    const auto index = sub_file_index(m_current_frame);
+    if (index != m_current_subfile) {
+        open_data_file(index);
     }
 
     if (header) {
@@ -79,4 +79,4 @@ void CtbRawFile::open_data_file(size_t subfile_index) {
     }
 }
 
-} // namespace aare
\ No newline at end of file
+} // namespace aare