Open circuit voltages (OCVs) for a PEM fuel cell operating at 3.0 atm, 100% relative humidity and in the temperature range of 23-120 degrees C were measured. It was observed that the OCV decreased with increasing temperature. Analysis of the partial O-2 and H-2 pressures in the feed streams revealed that the decrease in partial pressures with temperature were largely responsible for the decrease in OCV. The difference between the theoretical and measured OCVs was analyzed based on the literature, calculations, and hydrogen crossover measurements at OCV. It was concluded that the difference in OCV is caused mainly by two factors, one is the mixed potential of the Pt/PtO catalyst surface, and the other is hydrogen crossover. (c) 2006 Elsevier B.V. All rights reserved.