During PEM fuel cell operation, formation of H2O2 and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe2+, Pt2+) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nation). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nation decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nation membranes at conditions relevant to fuel cell environments (30-100%RH and 80 degrees C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe2+ ions prior to H2O2 exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H2O2-exposed membranes were found to strongly depend on Fe content and H2O2 treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H2O2 exposure. (C) 2010 Elsevier B.V. All rights reserved.