I don't know enough about your scenario to give you specifics, but if you have control of the source code for both domains, you can easily update the code to do what you need. If you don't have the ability to modify how anotherdomain.com works, then you will probably have to abstract the login process a bit. You would need to post back to your own login page, and inside of that make a request to the other domains login page, and handle the response. If you do have control of the code on both domains, and both are ASP.NET, then I think that link I posted earlier can be helpful, if not solve your problem outright.