As for keeping the 1.6 DLLs in the bin\ folder and the 1.5 DLLs in the GAC, it will work fine once you get rid of the pesky reference to the 1.5 that is hanging on somewhere. Most likely, it's in an obscure place somewhere (not in the web.config) in one of the <%Page %> declarations or something. Just do an "Entire Project" search and you should find something somewhere.