diff --git a/.github/workflows/cli-tests.yml b/.github/workflows/cli-tests.yml
index c57d739..2996bf4 100644
--- a/.github/workflows/cli-tests.yml
+++ b/.github/workflows/cli-tests.yml
@@ -2,8 +2,7 @@ name: CLI tests
 
 # Build ("compile") and test the Python command-line tool across Linux,
 # Windows, and macOS Apple Silicon (macos-14, running natively on arm64). The
-# tests use a fake sonpy shim, so they do not require CED's proprietary
-# binaries.
+# tests use a fake sonpy shim, so they do not require CED's sonpy binaries.
 
 on:
   push:
diff --git a/.github/workflows/matlab-tests.yml b/.github/workflows/matlab-tests.yml
index 4bf07f3..98ec906 100644
--- a/.github/workflows/matlab-tests.yml
+++ b/.github/workflows/matlab-tests.yml
@@ -3,7 +3,7 @@ name: MATLAB tests
 # matbox-style CI (mirroring VH-Lab/NDR-matlab) for the +sonpipe MATLAB
 # wrappers. Runs the matlab.unittest suite across Linux, Windows, and macOS
 # Apple Silicon (macos-14). The MATLAB tests drive a fake sonpipe CLI
-# (Python + numpy), so CED's proprietary sonpy binaries are not required.
+# (Python + numpy), so CED's sonpy binaries are not required.
 #
 # Licensing: matlab-actions provides MathWorks licensing for free on PUBLIC
 # repositories, so no token or secret is needed. (If this repo is kept private,
diff --git a/README.md b/README.md
index 86b45bd..0346ffd 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,8 @@
 # sonpipe
 
 A lightweight command-line **bridge** for reading Cambridge Electronic Design
-(CED) Spike2 data files. `sonpipe` extracts data from proprietary Spike2 files
-with CED's [`sonpy`](https://pypi.org/project/sonpy/) library and streams it as
+(CED) Spike2 data files. `sonpipe` extracts data from Spike2 files with CED's
+[`sonpy`](https://pypi.org/project/sonpy/) library (GPLv3) and streams it as
 **raw binary bytes** to standard output, so a host environment such as MATLAB
 can ingest it in chunks — quickly, predictably, and cross-platform.
 
@@ -24,11 +24,12 @@ functions that call the CLI for you.
    system call. It never shares MATLAB's memory space, so there are no version
    locks, environment conflicts, or interpreter crashes inside your workspace.
 
-2. **Licensing via pip.** CED's `sonpy` is proprietary. sonpipe does **not**
-   bundle or redistribute it; instead `pip install sonpipe` declares `sonpy` as
-   a dependency, so pip fetches the official, authorized CED binaries directly.
-   (On Apple Silicon, CED's current wheel is x86_64-only — see the Apple Silicon
-   note under Installation.)
+2. **Licensing via pip.** CED's `sonpy` is licensed under the GPL v3. sonpipe
+   does **not** vendor it; instead `pip install sonpipe` declares `sonpy` as a
+   dependency, so pip fetches the official build from PyPI. Keeping it a runtime
+   dependency (rather than bundling) leaves sonpipe's own MIT distribution free
+   of GPL copyleft. (On Apple Silicon, CED's current wheel is x86_64-only — see
+   the Apple Silicon note under Installation.)
 
 3. **No text-parsing overhead.** Waveforms and event times are written as raw
    little-endian binary, not JSON/CSV text. MATLAB captures the byte stream and
@@ -100,9 +101,12 @@ sonpipe --version
 This also installs `sonpy` (from CED, via PyPI) and `numpy`. For an isolated,
 PATH-managed command you can alternatively use `pipx install sonpipe`.
 
-> **Note on the CED license.** `sonpy` is proprietary CED software. It is
-> fetched by pip at install time and is intentionally not included in this
-> repository.
+> **Note on the `sonpy` license.** `sonpy` is CED software licensed under the
+> **GPL v3** and distributed by CED as prebuilt binaries (the underlying SON64
+> C source is not published). It is fetched by pip at install time and is
+> intentionally not bundled in this repository, so sonpipe's own MIT
+> distribution stays free of GPL copyleft. GPL places no restrictions on *use*
+> (reading your own files); obligations attach only to redistribution.
 
 ---
 
@@ -309,5 +313,5 @@ sonpipe/
 ## License
 
 sonpipe is released under the MIT License (see `LICENSE`). It depends on, but
-does not include, CED's proprietary `sonpy` library, which carries its own
-license.
+does not include, CED's `sonpy` library, which is licensed separately under the
+GPL v3.
diff --git a/matlab/+sonpipe/Contents.m b/matlab/+sonpipe/Contents.m
index cdb6839..fd65237 100644
--- a/matlab/+sonpipe/Contents.m
+++ b/matlab/+sonpipe/Contents.m
@@ -2,7 +2,7 @@
 %
 %   The +sonpipe package reads Cambridge Electronic Design (CED) Spike2 data
 %   files by calling the standalone "sonpipe" command-line tool, which in turn
-%   uses CED's proprietary sonpy library. Python runs in its own isolated
+%   uses CED's sonpy library (GPLv3). Python runs in its own isolated
 %   process; nothing is loaded into the MATLAB interpreter, so there are no
 %   version locks or interpreter crashes.
 %
diff --git a/pyproject.toml b/pyproject.toml
index 77e3015..033ab36 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,11 +19,11 @@ classifiers = [
     "Topic :: Scientific/Engineering",
 ]
 
-# NOTE: sonpy is a proprietary Cambridge Electronic Design (CED) package.
-# We declare it as a dependency so that `pip install sonpipe` legally and
-# automatically fetches the official, authorized sonpy binaries from PyPI/CED.
-# We deliberately do NOT vendor or redistribute sonpy in this repository, in
-# order to comply with CED's license.
+# NOTE: sonpy is Cambridge Electronic Design (CED)'s GPLv3-licensed Python
+# interface to the SON64 library. We declare it as a runtime dependency so that
+# `pip install sonpipe` fetches the official build from PyPI. We deliberately do
+# NOT vendor it in this repository -- keeping sonpipe's own MIT distribution
+# free of GPL copyleft, and letting users obtain sonpy directly from CED/PyPI.
 dependencies = [
     "numpy>=1.17",
     "sonpy>=1.7",
diff --git a/src/sonpipe/__init__.py b/src/sonpipe/__init__.py
index 6598161..847717d 100644
--- a/src/sonpipe/__init__.py
+++ b/src/sonpipe/__init__.py
@@ -1,15 +1,15 @@
 """sonpipe - a CLI bridge for reading CED Spike2 .smrx / .smr files via sonpy.
 
-sonpipe extracts data from proprietary 64-bit ``.smrx`` (and legacy 32-bit
-``.smr``) files produced by the Cambridge Electronic Design (CED) Spike2
-acquisition system and streams it, as raw binary bytes, to standard output.
-This lets a host environment such as MATLAB drive data ingestion in chunks and
-reinterpret the byte stream directly (e.g. with ``typecast``) without any
-text-parsing overhead.
+sonpipe extracts data from 64-bit ``.smrx`` (and legacy 32-bit ``.smr``) files
+produced by the Cambridge Electronic Design (CED) Spike2 acquisition system and
+streams it, as raw binary bytes, to standard output.  This lets a host
+environment such as MATLAB drive data ingestion in chunks and reinterpret the
+byte stream directly (e.g. with ``typecast``) without any text-parsing overhead.
 
-The proprietary reader, :mod:`sonpy`, is provided by CED and is fetched
-automatically by ``pip install sonpipe``.  It is intentionally *not* vendored
-in this repository, to comply with CED's license.
+The reader, :mod:`sonpy`, is CED's GPLv3-licensed interface to the SON64
+library and is fetched automatically by ``pip install sonpipe``.  It is
+intentionally *not* vendored in this repository, so sonpipe's own MIT
+distribution stays free of GPL copyleft.
 
 The command-line tools mirror the reading functions found in the NDR-matlab
 ``ndr.format.ced`` package:
diff --git a/src/sonpipe/sonfile.py b/src/sonpipe/sonfile.py
index d08b4b2..55e452f 100644
--- a/src/sonpipe/sonfile.py
+++ b/src/sonpipe/sonfile.py
@@ -4,7 +4,7 @@
 
 * the rest of sonpipe deals in plain integers, numpy arrays and dicts, and
 * tests can inject a fake ``sonpy`` implementation (see ``tests/fakesonpy.py``)
-  and run on machines where the proprietary CED binaries are not available.
+  and run on machines where CED's sonpy binaries are not available.
 
 Channel numbering
 -----------------
@@ -55,7 +55,7 @@ def _resolve_son_module(root, importer):
 def load_sonpy():
     """Import and return the sonpy module that exposes ``SonFile``.
 
-    ``sonpy`` is a proprietary CED package and is not vendored with sonpipe.
+    ``sonpy`` is CED's GPLv3-licensed package and is not vendored with sonpipe.
     """
     import importlib
 
@@ -66,7 +66,7 @@ def load_sonpy():
             "The 'sonpy' package (Cambridge Electronic Design) is required but "
             "could not be imported.\n"
             "Install it with:  pip install sonpy\n"
-            "sonpy is proprietary CED software and is fetched from PyPI on "
+            "sonpy is CED's GPLv3-licensed software, fetched from PyPI on "
             "install; it is intentionally not bundled with sonpipe.\n"
             f"(import error: {exc})"
         )
diff --git a/test/+sonpipe/+unittest/TestCase.m b/test/+sonpipe/+unittest/TestCase.m
index fa2cd71..a84e189 100644
--- a/test/+sonpipe/+unittest/TestCase.m
+++ b/test/+sonpipe/+unittest/TestCase.m
@@ -3,7 +3,7 @@
 %
 %   Points sonpipe.executable at a fake sonpipe CLI (fakecli.py) that runs the
 %   real sonpipe Python code against a synthetic in-memory file, so the MATLAB
-%   wrappers can be tested end-to-end without CED's proprietary sonpy binaries.
+%   wrappers can be tested end-to-end without CED's sonpy binaries.
 %
 %   Tests are skipped (filtered) if a working Python 3 with numpy is not found.
 
diff --git a/test/+sonpipe/+unittest/fakecli.py b/test/+sonpipe/+unittest/fakecli.py
index e38c1ae..4183263 100644
--- a/test/+sonpipe/+unittest/fakecli.py
+++ b/test/+sonpipe/+unittest/fakecli.py
@@ -2,7 +2,7 @@
 """A stand-in for the ``sonpipe`` command-line tool, for MATLAB unit tests.
 
 It runs the *real* sonpipe CLI code, but injects the test fake sonpy module in
-place of CED's proprietary ``sonpy``.  That way the MATLAB ``+sonpipe`` wrappers
+place of CED's ``sonpy``.  That way the MATLAB ``+sonpipe`` wrappers
 can be exercised end-to-end (argument construction, JSON decoding, binary
 typecast, channel dispatch, time-vector math) on any machine, without the CED
 binaries and without a Python/MATLAB in-process bridge.
diff --git a/tests/fakesonpy.py b/tests/fakesonpy.py
index 5f98357..1f01cda 100644
--- a/tests/fakesonpy.py
+++ b/tests/fakesonpy.py
@@ -1,9 +1,9 @@
 """A minimal fake of the CED ``sonpy.lib`` module for testing.
 
 It reproduces the parts of the ``sonpy`` API that :class:`sonpipe.SmrxFile`
-relies on, so the package can be exercised on machines without CED's
-proprietary binaries.  It is *not* a real SON file reader -- it serves synthetic
-in-memory data whose shape matches what real sonpy returns.
+relies on, so the package can be exercised on machines without CED's sonpy
+binaries.  It is *not* a real SON file reader -- it serves synthetic in-memory
+data whose shape matches what real sonpy returns.
 
 The channel layout of the default fake file (mirrors a small Spike2 file):
 
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 1ba46a5..49152a4 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -6,7 +6,7 @@
 
 They run when BOTH of the following hold:
 
-* the ``sonpy`` package (proprietary CED) is importable, and
+* the ``sonpy`` package (CED, GPLv3) is importable, and
 * a real Spike2 file is available -- by default the repository's
   ``example/spike2data.smrx``, or whatever ``SONPIPE_TEST_FILE`` points at.