The objective of this research work is to clarify essential factors related to degradation of PEFC electrodes. The degradation of Pt catalysts and the carbon support after a daily start and stop (DSS) operation was evaluated separately by measuring the electrochemical active area (Spt) and the double layer charging current (IDL) with cyclic voltammetry without and with CO adsorption on Pt catalyst surfaces, respectively. Corrosion of the carbon support was accelerated mainly by raising the operating potential to 0.90 V, which was observed by increase in IDL. The high-temperature operation at 90 degrees C or high-potential operation at 0.90 V resulted in a dramatic decrease in Sp, In contrast, it was found that the apparent cell performance was not so degraded probably because the decrease in Sp, was cancelled by an increase in the proton conductance in the swollen ionomer. The agglomeration of Pt catalyst particles and their migration in the electrolyte membrane were also analyzed by TEM observation after the DSS operations. (C) 2007 Elsevier Ltd. All rights reserved.