I tried your solution, but when debugging, the second dll file (office 2007 dll) is not copied to the Debug folder, only the first one (office 2000 dll) is copied. If I remove the reference to the first dll than the second one is copied. So it seems that only one of the dlls can be in the compiled result. I assume Visual Studio detects an Assembly name or something and this name is the same in both dll files. I managed to get both dll files in there by adding them both manually and setting build action to "None" and Copy to Output Directory to "Copy Always". The problem is that when debugging, the program crashes with an error that the second dll file could not be found. :sigh: I'm out of options at the moment. Hope you can help me.