The dependence of the oxygen reduction reaction (ORR) activity of Pt/c catalysts used in actual polymer electrolyte fuel cell membrane electrode assemblies (PEFC MEAs) on cell temperature, O-2 partial pressure and catalyst loading has been investigated. Based on physical chemistry and electrochemistry points of view, the charge transfer coefficient (a) was demonstrated to vary with potential over a narrow range or 0.5-0.6. The double Tafel slope occurring at potentials above 0.8 V vs. RHE was considered to be caused fly the effect or adsorption or reaction of oxygen-containing species sourced by water. As a result, the alpha was assume constant of 0.5 irrespective or the potential, and used fit the estimation of the ORR activity [A/cm(2)-Pt]. The ORR activity was observed to increase linearly with increasing temperature. It was also round to be dependent on O-2 partial pressure (considered as O-2 concentration) until it reached a plateau at an O-2 concentration of 40%, and then remained almost constant. This finding implies that in gas-phase PEFC MEAs, adsorption or O-2 as a reactant can influence and govern the electrochemical reaction. In addition, the ORR activity is demonstrated to be independent of the Pt/C ratio of the catalyst not only for Pt plate electrodes in liquid-phase systems, but also for MEA electrodes in the gas-phase system of actual single-cell PEFCs. The ORR activity was found to be almost constant at 2.6 x 10(6) A/cm(2)-Pt at 60 degrees C for Pt/C ratios from 19.3 to 55.8 wt%.