A 2700 h life test of a single proton exchange membrane fuel cell (PEMFC) with Nafion 112 membrane under nonsaturated humidification was conducted. In situ measurements of hydrogen crossover rate through the membrane and electrochemical active surface area (EAS) of Pt catalyst, in combination with cell polarization curves, were applied to investigate the degradation mechanism. A slow decrease of the EAS of Pt catalyst from 400 to 1900 h running was found to cause a slight voltage decay only at high current densities. However, a subsequent dramatic degradation of the membrane results in an accelerated collapse of the fuel cell. (C) 2005 The Electrochemical Society.