The durability of perfluorinated ionomer membrane, Nafion (R) 117, with various counter ions against hydrogen peroxide was investigated as a degradation factor of polymer electrolyte fuel cells (PEFC). After protonated Nafion membrane was tested in 30% H2O2 solution at 80 degrees C, small amounts of fluoride ion and sulfate ion, which are derived from the C-F bonds and the sulfonic acid groups, respectively, of the membrane, were detected in the solution. This fact indicated a potential vulnerability of the electrolyte membrane to H2O2 formed in the cell. The durability of Nation with alkali and alkaline earth metal ions as counter ions were similar to that of protonated Nation, and hence these cations do not have any specific effects on membrane degradation. In contrast, the presence of ferrous and cupric ions as counter ions significantly enhanced the decomposition rate of Nation. This is due to the formation of strongly nucleophilic radicals such as hydroxy and hydroperoxy radicals upon decomposition of H2O2 at these catalyst ions. The results of FT-IR and F-19 NMR measurements of deteriorated Fe2+-Nation membrane revealed that both the main and side chains are decomposed at similar rates by radical attack, most probably because the decomposition proceeds through radical de-polymerization (so-called un-zippering mechanism). (c) 2005 Elsevier B.V. All rights reserved.