Isentropic efficiency is a common performance measure for compressors and is useful for modelling the behaviour of real compressors in relation to discharge temperature or required mechanical power input. However, it has a weakness in that a basic assumption in the calculation is that the real compressor is adiabatic. If significant heat transfer from the compressor to the outside environment occurs, then the adiabatic assumption is invalid, but the negative effects of internal entropy generation are reduced, and in principle it is possible to have a measured isentropic ‘efficiency’ greater than 100%, which, while counterintuitive, is highly desirable. We practically demonstrate this effect with an air-cooled swashplate compressor and propose that for cases of intentional compressor cooling, the simple definition for isentropic efficiency is retained as a performance measure but renamed to isentropic performance coefficient (IPC).