The GAC in .NET 4.0 is cut into two parts now -- one half for .NET Framework 2.0 stuff, and the other for v4.0 stuff. When compiling any project, targeting .NET 2.0, 3.5, or what have you, this annoying error may pop up: Could not load file or assembly 'System.EnterpriseServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d5... or ......