Formation of radicals, such as HO center dot, H-center dot and HOO center dot, in the membrane of the polymer electrolyte fuel cell and their attack on perfluoroalkylsulfonic acid (PFSA) and poly(styrenesulfonic acid) (PSSA) ionomers was simulated based on a kinetic framework with H2O2 as "parent" molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10(-19) M for H-center dot, 10(-16) M for HO center dot and 10(-10) M for HOO center dot. H-center dot is formed via the reaction of HO center dot with H-2 dissolved in the membrane. The attack of the PFSA ionomer was assumed to proceed via weak carboxylic end-groups. The corresponding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test conditions. The predicted FER under fuel cell operating conditions was underestimated by 2-3 orders of magnitude. It is likely that degradation via side-chain attack is prevalent during open circuit voltage hold tests. The oxidative degradation of PSSA ionomer follows an entirely different pathway, because, in addition to alpha-hydrogen abstraction by HO center dot, the aromatic ring effectively scavenges HO center dot to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3581040] All rights reserved.