 <p>Try to access it via the console application with no development tools installed (aka, a live environment or a development server which do not have VS2008 installed) and try again (RE: the comment posted by Heinzi). </p> <p>If you still can then check to see if your web app is running under the correct .NET framework version (and that it's installed on the server).</p> <p>Since this appears not to be an ISAPI DLL requiring 32-bit application to be switched on, but a C# Web App. My money is there is something wrong with the web app deployment on the server and not the DLL itself.</p> <p>Also remember, if the DLL is not .NET, you need to register it on the server using regsrv32 before you can access it with your web app anyway.</p> <p>Hope I helped.</p> <p><em>edit</em></p> <p>If all other attempts fail, try to re-compile the DLL you're using setting the target to X86 on the compiler and re-reference it in your web app.</p>