A mechanical fatigue life analysis of the type used for lifetime predictions in structural materials has been adapted to characterize PEM fuel cell membranes. It is shown that a stress vs. cycles-to-failure (S-N) curve analysis is capable of: (1) predicting the mechanical durability of PEM membranes under humidity cycling, and (2) assessing the fraction of membrane life remaining in degraded MEAs. Mechanical fatigue testing was performed in a dynamic mechanical analyzer (DMA) at 60 degrees C and 90% relative humidity (RH) under different values of stress amplitude, and the results were used to plot the S-N curve. The stress amplitude for the S-N curve was then mapped to the equivalent change in RH as a methodology for characterizing membrane mechanical resistance to humidity cycling. (c) 2010 Elsevier B.V. All rights reserved.