diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -225,6 +225,15 @@
 
    Availability: Unix.
 
+   .. note:: On MacOSX the getgroups function behaves different from other
+      Unix platforms. The getgroups function always returns the list of groups
+      the user was a member of when the process was started, even when 
+      :func:`setgroups` was called to change the list. 
+
+      This only affects MacOSX systems where the python binary was compiled
+      with a deployment target of 10.5 or later, binaries targetting 10.4
+      or earlier behave like other unixes.
+
 
 .. function:: initgroups(username, gid)
 
@@ -412,6 +421,14 @@
 
    Availability: Unix.
 
+   .. note:: On MacOSX the maximal number of supplemental groups in the *groups*
+      argument is 16, even though :func:`getgroups` can return more than 16
+      groups.   Futhermore, calls to :func:`getgroups` will not reflect the 
+      updated list, although the system will honor the new setting.
+
+      This only affects MacOSX systems where the python binary was compiled
+      with a deployment target of 10.5 or later, binaries targetting 10.4
+      or earlier behave like other unixes.
 
 .. function:: setpgrp()