We recently ran into a problem where the aspnet_merge process was failing. The default output messages and warnings simply say that the merge failed and that the program exited with an error code of 1. We couldn't figure out why the merge was failing because the information in the output window seemed OK. We found that you have to increase the verbosity for the output window to reflect what actually happens during the merge process. This setting is for MSBuild. Go to [tools], [options], [build and run] and set the [MSBuild project build output verbosity] to "Detailed". That will tell you specific error messages generated during the build and merge process.