Currently, the energy crisis and global warming are two urgent global problems. Because of its high efficiency and low emissions, a proton exchange membrane fuel cell (PEMFC) has become a promising power source candidate. However, the stability and lifetime of this cell remain to be addressed before it can be fully developed. In this paper, we discuss the sequential acidity change of the PEMFC product. Two operating factors (cell and fuel temperature) with three levels are designed for these experiments. The fuel gas with the fully humidified feeding flow was oxygen (140 sccm) in the cathode and hydrogen gas (210 sccm) in the anode. The product water that drained out of the PEM fuel cell cathode outlet was measured by a pH meter in real time. The sequential water acidity varied case by case, and its pH value ranged from 3 to 7. Furthermore, membrane degradation enhanced water acidity owing to the abundant production of the hydroxyl free radical (OH)-O-center dot. The more acidic product severely corrodes the metallic material and shortens the lifetime of the fuel cell. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.