A fluorescence probe method, which enables highly sensitive detection of OH radicals (center dot OH) by forming a stable fluorescent product, was applied to a model reaction system of polymer electrolyte fuel cells (PEFCs). center dot OH was successfully detected under open-circuit condition with a homemade test cell having reservoirs of a coumarin probe solution at both anode and cathode electrodes. The experiments with H2O2 and Fe2+ in the probe solution suggested that the center dot OH is produced via electrode reduction and oxidation of H2O2 at the anode and cathode, respectively.