The performance of a proton exchange membrane fuel cell (PEMFC) under various operating conditions was investigated experimentally using dry and humidified hydrogen and oxygen as reactant and oxidant gases, respectively. Experiments have been carried out on a single PEM fuel cell with the active area of 5 cm(2). The results showed that the most important factor affecting the performance of PEMFC is the mass transport limitation including the transport of reactant and oxidant gases to active sites of catalyst layer, the transport of the proton from the anode side to the cathode side through the membrane, and the transport of produced water from the cathode side to the anode side by back diffusion mechanism. Operating parameters that examined in this paper i.e. temperatures, pressures, and humidity of reactant gases could decrease these limitations and improve the performance of the fuel cell. Based on these investigations, the optimum conditions are operation at higher pressure and elevated temperature with the humidified reactant gases. Furthermore, pressurized cathode side has better affected on the performance of the PEM fuel cell than pressurized anode side. (c) 2006 Elsevier B.V. All rights reserved.