calling QueryInterface on a COM object

I've got a third party COM object I'm using, and I need to get a different interface for one of the objects. In COM this is done by calling QueryInterface() on the COM object with a REFIID. I'm told this is a GUID, but when I try the appropriate GUID I get "Member not found" regardless of how I try to pass it. Has anyone successfully retrieved an alternate interface from a COM object? Thanks, --Travis