Changes in a perfluorosulfonated acid polymer membrane in membrane electrode assemblies were studied after different times under stationary conditions in fuel cells. A large series of characterizations demonstrated changes in the morphology, mechanical behavior, and thermal stability upon aging. Overall, the membrane evolution could be mainly attributed to both chemical degradation and cationic contamination. The reduction in the membrane thickness, detected by scanning electron microscopy, was ascribed to a radical unzipping mechanism and polymer chain erosion after 900 h in service. An additional monotonic decrease in the number of CtertiaryF groups was observed even at 400 h. In parallel, membranes were cation-contaminated, and this led to drastic changes in the thermal and mechanical properties in the first stage of fuel-cell operation. The pollution cations were shown to have Lewis acid strengths close to 0.25 and thus strongly interacted with sulfonate anions of the membrane. The kinetic dependence of these membrane modifications and the influence of the platinum band were also examined. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2121-2132, 2010