diff -r b0ef6a5a6dcc Doc/library/ctypes.rst
--- a/Doc/library/ctypes.rst	Fri Sep 30 08:46:25 2011 +0300
+++ b/Doc/library/ctypes.rst	Fri Sep 30 10:12:39 2011 -0700
@@ -633,6 +633,17 @@
    <Field type=c_long, ofs=0:16, bits=16>
    >>>
 
+It is important to note that bit field allocation and layout in memory is not 
+defined as a standard, rather its implementation is compiler-specific. On Windows, 
+Python uses the same bit field allocation as Microsoft VC++ compiler. On Unix 
+systems, it uses the same bit field allocation as GCC. This enables 
+interoperability with libraries compiled with VC++ on Windows and GCC on Unix.
+
+It is possible that the exact same Structure declaration containing a bit field 
+will have different sizes in memory on Unix and Windows systems, so it is 
+recommended that no assumptions are made about the structure size and layout. 
+For example, accessing the raw memory allocated for the Structure to retrieve 
+data will yield different results on different operating systems.
 
 .. _ctypes-arrays: