The oxygen reduction reaction (ORR) catalyzed by Pt was studied in the presence of Co2+ using cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE) techniques in an effort to understand fuel cell cathode contamination caused by Co2+. Findings indicated that Co2+ could weakly adsorb on the Pt surface, resulting in a slight change in ORR exchange current densities. However, this weak adsorption had no significant effect on the nature of the ORR rate determining steps. The results from both RDE and RRDE indicated that the overall electron transfer number of the ORR in the presence of Co2+ was reduced, with similar to 9% more H2O2 being produced. We speculate that the weakly adsorbed Co2+ on Pt could react with the H2O2 intermediate and form a Co2+-H2O2 intermediate, inhibiting the further reduction of H2O2 and thus resulting in more H2O2 production. The fuel cell performance drop observed in the presence of Co2+ could be attributed to the reduction in overall electron transfer number and the increase in H2O2 production. Higher production could intensify the attack by H2O2 and its radicals on membrane electrode assembly components, including the ionomer, carbon support, Pt particles, and membrane, leading to fuel cell degradation. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.