The formation of OH radicals ((OH)-O-center dot) for polymer electrolyte fuel cell (PEFC) was investigated with a home-made test cell by means of a fluorescence probe method for several kinds of membrane electrode assembly (MEA) consisting of Nation. After 12-h operation under open circuit condition, (OH)-O-center dot was detected at both the anode and the cathode sides but the amount was much larger for the anode side. The formation of (OH)-O-center dot considerably depended on the amount of Pt/C catalyst loaded in the MEA. Dispersion of Pt particles in Nation membranes increased the (OH)-O-center dot formation, but Pt ions did not. Based on the results, the formation mechanism was discussed. (C) 2010 Elsevier B.V. All rights reserved.