To clarify the deterioration mechanism for polymer electrolyte fuel cells, OH radical formation at the catalyst electrodes was investigated by ESR (electron spin resonance) spectroscopy using a flow cell with the catalyst electrodes. OH radicals produced from H2O2 were detected by a DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trapping method for a Nafion-coated Pt/Carbon catalyst electrode under a high potential (0.85 V versus RHE) on supplying H-2 and under low potentials (lower than 0.40 V). When Pt-Ru catalysts were employed instead of Pt catalysts, the formation of OH radicals was barely detected. The results suggest the possibility of the formation of OH radicals by the oxidation of H2O2 at the oxidized Pt surface under a positive potential as well as the reduction of H2O2 at the clean Pt surface under a low potential.