A series of single-cell, hydrogen-air proton exchange membrane fuel cells (PEMFCs) was operated for different lengths of time, namely, 200, 500, 700, and 1000 h. A group of reproducible and identical membrane electrode assemblies (MEAs) was used for those tests. Cell performance was studied by examining the cell polarization curves. After various lifetime tests, each MEA was cross-cut and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy, and Raman techniques to investigate any changes in catalyst structure and morphology, as well as particle size and chemical composition. The average particle size of the catalysts was calculated from XRD results and was found to increase with cell operating time. In addition, the agglomeration in nanometer-sized catalyst particles was observed from TEM analysis after prolonged cell operation. Ruthenium oxide was identified from Raman spectra of the anode catalyst from the tested MEAs, while no oxides were found on the cathode catalyst at the cell operating voltage. It is possible that the formation of metal oxides at the surface of the anode catalyst led to larger particles and ultimately resulted in the decrease of catalyst activity. This might be responsible for the slightly degraded cell performance following 700 h of operation. (C) 2003 The Electrochemical Society.