Nonfluorinated sulfonic acid membranes are a group of promising candidate materials for the commercialization of proton exchange membrane fuel cell (PEMFC) technology. However, one of the main obstacles is that the harsh fuel cell environment may originate different modes of degradation and aging processes that result in either chemical or morphological alteration in these membranes. The effect of peroxide radicals on PEM durability is of particular interest because a common feature of many hydrocarbon-based membranes is that the building block consists of sulfonic acid-substituted aromatic rings, which are much more sensitive to radical attack than the Teflon-like backbone in perfluorinated sulfonic acid type materials. In this work, we attempt to provide answers to the hydroxyl radical initiated durability issues at the PEM and electrocatalyst interface by analyzing the performance of two novel membranes, sulfonated poly (arylene ether sulfone) and sulfonated poly (ether ether) ketone, using a newly designed durability evaluation method under fuel cell-like conditions. This method is able to separate the membrane evaluation process into cathode and anode aspects. Under experimental conditions in this work, degradations in SPES-40 samples were found happening at the cathode (oxygen) side of the PEMFC. (C) 2006 The Electrochemical Society.