diff --git a/RELEASE_NOTES.rst b/RELEASE_NOTES.rst
index 4159ddb14..b3e5747f0 100644
--- a/RELEASE_NOTES.rst
+++ b/RELEASE_NOTES.rst
@@ -47,6 +47,7 @@ All changes
 
 - Document the :ref:`minimum version of Java <install-java>` required for :class:`ixmp.JDBCBackend <ixmp.backend.jdbc.JDBCBackend>` (:pull:`962`).
 - Improve type hinting (:pull:`963`).
+- Fix capitalization in auxiliary_settings.gms to enable GDX output file compression on MacOS and Linux. (:pull:`965`) 
 
 .. _v3.11.1:
 
diff --git a/message_ix/model/MESSAGE/auxiliary_settings.gms b/message_ix/model/MESSAGE/auxiliary_settings.gms
index 727a9b7e1..0df67fcae 100644
--- a/message_ix/model/MESSAGE/auxiliary_settings.gms
+++ b/message_ix/model/MESSAGE/auxiliary_settings.gms
@@ -26,7 +26,7 @@ option MCP = PATH ;
 
 *option solveopt=clear ;# remove results of previous runs in memory
 option solveopt=merge ; # keep results of previous runs in memory
-$SETENV GdxCompress 1   # reduces the size of the gdx export file
+$SETENV GDXCOMPRESS 1   # reduces the size of the gdx export file
 
 %calibration%$ONTEXT
 $ONLISTING