I've just created a ticket: https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/144 to suggest that we strengthen the ordering guarantees between sends (and receives) from different threads. I think that the real question here is whether the potential benefit to the user outweighs the potential for optimization in a multithreaded implementation. Please have a look at it and feel free to comment. Thanks, -d