This exception can occur when a .NET application running on a 64-bit platform attempts to load a DLL that does not support native 64-bit execution. If the application is built in platform-agnostic (i.e. "Any CPU") mode, it will launch in 64-bit mode on the host operating system, leading to this problem. To fix this, you can force the application to run in 32-bit mode by compiling for x86 architecture explicitly, forcing the program to launch in WoW64 mode on the 64-bit system, and allowing the 32-bit DLL to be accessed.

Had a problem when executing a dll and searched high and low on why it was erroring. I knew it wasn't a missing supporting DLL because the same DLL executed fine from a VB6 application. Recompiled the application targeting x86 and everything works great!

Thanks for the heads up Logan1337. Makes sense after thinking about it, but just assumed if ran ok under x64 that dlls would be fine. Not so! I also see why its taking so long for 64 bit apps to come out if you have to recompile every DLL to x64 as well.

I can't make the WCF host to treat my 32 compiled server.... still looking for a solution.. when will we have Edit and Continue in x64 ?

I got this exception when an executable (manged code) built with Any CPU tried to load a dll build with x86 on Vista 64. I had to rebuild that dll with Any CPU and this fixed it.

See more info here: http://www.hanselman.com/blog/BackToBasics32bitAnd64bitConfusionAroundX86AndX64AndTheNETFrameworkAndCLR.aspx