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