Degradation of the Nafion electrolyte in Proton Exchange Membrane Fuel Cells (PEMFCs) limits the lifetime, motivating development of materials that resist degradation. The mechanism for degradation of Nafion under fuel cell conditions remains uncertain. Studies of Nafion degradation in concentrated OH center dot environments, such as Fenton or vapor HOOH tests, show that the main chain significantly degrades in these conditions. However it has not been established whether this applies to fuel cell conditions. We have used quantum mechanics (Density Functional Theory with the B3LYP and M06 functionals) to determine the mechanism of Nafion degradation under both concentrated OH center dot and fuel cell conditions. These studies confirm that under concentrated OH center dot conditions Nafion degrades when peroxide radicals attack end groups (-COOH, -CF CF2, -CF2H); followed by degradation of Nafion along the polymer main chain, as proposed previously. However we find that under fuel cell conditions, Nafion degradation occurs along the polymer side chain starting with H center dot attacking the side chain groups such as the sulfonic acid, -503-. We find that it is easier for OH center dot to attack the main chain than H center dot, while vice versa, it is easier for W radical to attack the side chain than OH center dot. (C) 2013 Elsevier B.V. All rights reserved.