The influence of Pt: Pd atomic ratios (1:2-1:8) on a carbon support upon its suitability as a cathode for a proton exchange membrane (PEM) fuel cell was evaluated at a constant membrane electrocatalyst loading of 0.15 mg/cm(2). The results clearly demonstrated that the different Pt: Pd atomic ratios had a significant effect on both the electrocatalyst activity and also on the performance in a H(2)/O(2) fuel cell. Decreasing the Pt: Pd atomic ratio led to an increase in the particle size of the electrocatalyst but a decrease in the particle dispersion and electrochemical surface area (ESA). With respect to the performance in a PEM fuel cell, decreasing the Pt: Pd atomic ratio led to a decreased exchange current density (j(0)), electrocatalytic activity and also mass activity (M(A)), but to an increased total resistance (R) of the cell. The maximum activity of the oxygen reduction reaction (ORR) and the peak power (492 mW/cm(2)) were obtained with an electrocatalyst with a Pt: Pd atomic ratio of 1:2. Finally, the rotating disk electrode (RDE) analysis showed that the mechanism of oxygen reduction on the prepared Pt Pd/C electrocatalyst involved a four-electron pathway with high oxygen permeability in the Nafion film. (C) 2010 Elsevier Ltd. All rights reserved.