Various amounts of 20 wt % Pt/C catalysts (56.7-5.7 mu g(carbon) cm(-2)) were loaded on glassy carbon (GC) disk electrode, and the effect of agglomeration on hydrogen peroxide formation in oxygen reduction was investigated by the rotating ring-disk electrode technique. The formation of H2O2 was enhanced with a decrease in agglomeration of Pt/C. Even in the operating potential range of polymer electrolyte fuel cell cathodes (0.6-0.8 V), 10% hydrogen peroxide was formed at 5.7 mu g(carbon) cm(-2) Pt/C loaded on GC. These results revealed that series two-electron reduction pathway, which is negligible on clean bulk Pt surface, does exist on Pt particles supported on carbon. (C) 2004 The Electrochemical Society.