The efficacy of CeO2 nanoparticles in mitigating free-radical-induced polymer electrolyte membrane (PEM) degradation is investigated. Commercially obtained CeO2 and nanoparticles synthesized in-house were incorporated within a recast Nafion membrane. Membrane electrode assemblies were prepared using Nafion and Nafion-CeO2 composite membranes (0.5, 1, and 3 wt % CeO2). The composite membranes exhibited very similar proton conductivities (similar to 35 mS/cm) and hydrogen crossover (similar to 1 mA/cm(2)) as Nafion. However, the fluoride emission rate (from accelerated tests) was lowered by more than 1 order of magnitude upon addition of CeO2 into the Nafion membrane, suggesting that CeO2 nanoparticles have tremendous potential to greatly enhance membrane durability. (C) 2008 The Electrochemical Society.