The mechanism underlying the decline in cell voltage over time was investigated for high-temperature proton exchange membrane fuel cells. Five identical cells were prepared and long-term powergeneration tests were conducted at an operation temperature of 150 degrees C and a current density of 0.2 A cm(-2) for periods of up to 17,860 h. Each of the cells was then analyzed using transmission electron microscopy and electron probe micro-analysis (EPMA). The results indicated that growth of the Pt catalyst particles occurred during operation, in addition to oxidation of the carbon support. Degradation of the catalyst layers was investigated by EPMA of cross sections of the membrane electrode assemblies, allowing the mechanism of cell performance reduction to be clarified. (C) 2012 Elsevier B.V. All rights reserved.