An important challenge for proton exchange membrane fuel cells (PEMFCs) is the stability and durability of the membrane separator. In this study, fluoride ion emission rates were determined front fuel cell durability tests at different temperatures. Degraded membranes were removed after each test, and both the anode and cathode sides of degraded membranes were analyzed by X-ray photoelectron spectroscopy (XPS). The results show that degradation was accelerated by high temperature and that the decomposition of both sulfonic acid side chains and the poly(tetrafluoroethylene) backbone occurred. The degradation was more severe on the anode side, showing that at least half of the side groups were decomposed there. A multilayer membrane electrode assembly was also used to study degradation at different locations especially in the bulk membrane, which illustrates that the degradation also occurs there but is slower than that at the anode side. This result also indicates that the rate of membrane degradation slows with thicker membrane. Finally, an improved kinetic model of membrane degradation was proposed to explain the experimental results. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3187731] All rights reserved.