I've managed to set up compression for a custom mime type on IIS 7.5 with no problem. Add the mime type to IIS then add it to the httpCompression element in applicationHost.config. But when I do the same thing on IIS 7, the particular mime type is never compressed. This isn't a problem with compression in general since other mime types are compressed correctly. As far as I can tell, IIS 7 and IIS7.5 are configured in exactly the same way. Does IIS7 behave differently and do I need to do something else to get it working? I've setup failed request tracing and get a NO_MATCHING_CONTENT_TYPE error during compression but I can't figure out what else I need to do to tell IIS about my mime type