You can't do that unless the browser is closed.

If the browser isn't closed after the first session ends,
the Session ID will remain the same in the new session.

Juan T. Llibre, ASP.NET MVP
ASP.NET FAQ : http://asp.net.do/faq/
Foros de ASP.NET en Español : http://asp.net.do/foros/
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RC - From your question, I'm not sure exactly what you're doing but in the
current (nov 05) issue of the msdn, web q&a page 11, someone asks "if they
can force a new session id"? The editor suggest that this can be done by
editing the web.config file so that you have a cookieless session state:

=-=-=-=-=-
   <sessionState ... cookieless="true" ...  />
=-=-=-=-=-=-

In the cookieless mode, the server automatically inserts the sessionid into
the initial and all subsequent, relative url's. If you specify an absolute
URL for a redirect or popup a new browser session, a new sessionid will be
created (you'll see it as part of the url). YMMV - I personally am not a big
fan of having the sessionid in the url.