Even though the measured open-circuit voltage in a H-2-O-2 PEM fuel cell is invariably about 200-250 mV lower than that predicted from thermodynamics(1.229V at 25 degrees C), there is no unequivocal explanation of this phenomenon available in the literature, although several hypotheses exist. Based on a theoretical model of mixed potential with a priori parameters, it is shown here that this voltage loss under open-circuit conditions can be attributed exclusively to hydrogen crossover and the resulting oxygen reduction reaction overpotential at the cathode. The analytical model predictions agree well with available experimental results. (C) 2009 Elsevier B.V. All rights reserved.