Degradation and recovery of electrode performance during the operation of polymer electrolyte fuel cells (PEFCs) with sulfonated polyimide (SPI-8) and Nafion (R) membranes were evaluated by changes in cell voltage (E(cell)), mass activity (MA), Tafel slope (TS), and electrochemical surface area (ECA) of the Pt catalyst. During continuous cell operation, values of E(cell), MA and ECA decreased, but TS increased. It was found by ion chromatography combined with mass spectrometry (IC/MS) that the drain water contained, as decomposition products, bisulfate and several other ionic compounds derived from the side chain of SPI-8. These ionic compounds were found to adsorb on the surface of the Pt cathode catalyst and to suppress oxygen adsorption, resulting in increases in the overpotential for the oxygen reduction reaction (ORR). The cathode performance was able to recover by potential cycling under high humidity conditions, as the ionic compounds were removed from the Pt surface. (C) 2008 Elsevier Ltd. All rights reserved.