Interesting to see an MSDN blog that claims to demonstrate interoperability between an object serialised by a Java app and a J# app de-serialising it. Sample code included. Of course you can't use RMI between Java and J#, but you could exchange the object via HTTP-GET, a message service (via JMS), web services or even the filesystem. It's not clear if there are limits to this interoperability, but it's an interesting achievement. I'm sticking with Swing for desktop apps so that I can use Java end-to-end, but it's nice to know there are some ways for a .NET client to interoperate say with J2EE.