Compressed air plays a major role in industry throughout the world, but there currently exists no internationally accepted standard which provides a systematic procedure for the determination of the energy efficiency of air compressor systems. International Standard ISO 1217 (Displacement compressors – Acceptance tests) covers a very wide range of compressors, but is, of necessity, too general to be used as a means by which the energy performance of specific types of plant such as packaged compressors may be determined, with the expectation that comparable efficiency figures be obtained by laboratories throughout the world. It lacks, for example, specific requirements for a suitable measurement system layout, acceptable test conditions including ambient air temperature and barometric pressure limits, recommended ‘run-in’ and temperature stability requirements and guidance as to suitable instrumentation, including, most importantly, the measurement of air flow. The results of a  ‘round robin’ procedure in which two packaged air compressor units have been tested for energy efficiency at  laboratories in Germany, Denmark and Australia are presented. It has been found that the most critical part of the required measurements are those of air flow, and recommendations are made as to the most appropriate types of flow-meter. Calibration of flow measuring equipment is also addressed, and suggestions made, how calibration may be performed in situ, using the compressor test rig itself, enabling verification of flow meter performance. The primary aim has been to distill the results and experiences of the three laboratories and to compile these into a Guide to the use of ISO 1217, including a spreadsheet calculator to provide a final isentropic efficienciues, so that by following its instructions, compressor performance obtained by any competent laboratory could be close.