Investigating the degradation mechanism of a polymer electrolyte fuel cell (PEFC) is indispensable for realizing a longer-lasting fuel cell. This report describes the research results that the cathode catalyst after a 9000h cell operation has been removed to evaluate the O-2 reduction reaction (ORR) and H2O2 generation amount during the ORR. First, the power generation performance of the prepared PEFC single cell was measured vs. the elapsed time of the long-term test operated at 80 degrees C at the constant current density of 200 mA cm(-2). After the 9000 h operation, the power generation performance was confirmed to decrease. Subsequently, the cathode material containing the Pt-Co/C and the ionomer was removed from the cell and packed into the microcavity of the porous microelectrode (PME). By using the PME, ex situ electrochemical measurements were done in a sulfuric acid solution. The background cyclic voltammogram current of the 9000h sample was one order of magnitude lower than that of the Oh sample, suggesting that the electrochemical surface area had decreased. The diffusion-limiting ORR current decreased after the endurance test, however, the Tafel slopes were the same between the two in the same electrode potential region. This revealed that the endurance test does not affect the electron transfer mechanism of the ORR. By using the scanning electrochemical microscopy (SECM), the H2O2 yield assessed for the 9000 h sample at 0-0.7 V vs. RHE during the ORR was found to be 3-4 times higher than that for the Oh sample. Based on the TEM observation, the high H2O2 yield after the 9000 h operation was considered to be due to the degradation of the ionomer incorporated in the cathode layer with the Pt-Co/C catalyst. (C) 2013 Elsevier Ltd. All rights reserved.