The performance of a polymer electrolyte membrane fuel cell (PEFC) power stack is assessed in terms of polarization measurements under laboratory conditions by applying a harmonized testing procedure. Harmonization at the level of testing procedures allows for an objective and trustworthy comparison of performance data. The harmonization of the procedure took place among the 55 partners of the Fuel Cell Testing and Standardization Network (FCTESTNET). Selected testing procedures are currently validated through experimental campaigns in the successor project (Fuel Cell Testing, Safety and Quality Assurance, FCTESQA) involving research laboratories from Europe, US, China, and Korea. This study reports the results of a test of the campaign for the performance assessment of a PEFC power stack carried out at JRC by applying the harmonized procedure for validation. Following the procedure, the test inputs and outputs are subjected to stability checks a priori and during the actual polarization measurement steps of the test. The assessment of the stack performance is based on a statistical approach of the test outputs, which includes the calculation of averages, measurement range, (relative) standard deviation and standard error as well as of a stability parameter. The study demonstrates the necessity of harmonized testing procedures and of a harmonized methodology of presenting the test results in a commonly agreed format to provide a comprehensive performance assessment for PEFC stacks. Copyright (C) 2011 John Wiley & Sons, Ltd.