OH radicals formed from H2O2 (at 0.0-1.3 V vs a reference hydrogen electrode) and O-2 (at 0.0 V) on the Pt/C catalyst electrodes for polymer electrolyte fuel cells (PEFCs) were successfully detected by electron spin resonance (ESR) spectroscopy after improvement of the detection system to prevent the decomposition of 5,5-dimethyl-1-pyrroline N-oxide spin-trapping reagent. Because under the deaerated condition OH radicals were not detected, the experimental result is consistent with the proposed degradation mechanism of PEFCs that OH radicals are produced at the anode side by the crossover of O-2 gas from the cathode side.