Hey guys: Had a good talk w/ Dan and Lincoln at the conclusion of JBossWorld regarding wire compatibility and was asked to pass references along to this list. The key is that marking a class as "implements Serializable" isn't sufficient to: 1) Make it really Serializable 2) Ensure that the wire protocol maintains compatibility across releases I've been using a technique documented by Bob Lee awhile back: http://crazybob.org/2006/01/unit-testing-serialization-evolution_13.html The idea is that after you've formalized your protocol (ie. Externalizable or readObject/writeObject explicitly), you make a copy of this original class. Then, using a special ObjectOutputStream, you can redefine this class as a new name, hence converting from old to new or new to old. For instance, I implement this here in ShrinkWrap to make Archives Serializable: http://anonsvn.jboss.org/repos/common/shrinkwrap/trunk/impl-base/src/test... Recommend this approach, or one similar. Some folks like to serialize the original version into a .ser file which stays in SCM, and ends up being the basis for the test point. But that can only test compat in one direction. S, ALR