The effect of CeO2 and ZrO2 on the durability of perfluorinated sulfonic acid ionomer membranes in proton exchange membrane fuel cells is widely discussed in the literature and their effectiveness is shown. However, in-situ fuel cell testing is a time consuming, costly process, and it is beneficial to develop representative ex-situ analyses that allow for rapid screening and ranking of additives in their ability to mitigate degradation. In this work, we investigate the reaction of perfluorosulfonic acid (PFSA) ionomer to radicals generated using Fenton's reagent and draw the conclusion that this widely-employed ex-situ degradation technique leads to an extraordinarily large and unrealistic accumulation of iron species in the membrane. In response, we developed an ex-situ degradation procedure in which membranes are cast from Nafion (R) D2020 solutions containing FeSO4 and subsequently exposed to only aqueous H202 solution (thus maintaining a constant, representative Fe2+/3+ content in the membrane). Nafion (R) D2020 was also co-cast with up to 10 wt% of CeO2, ZrO2 and yttria-stabilized zirconia (YSZ). Using this protocol, it is found that CeO2-containing membranes reduce fluoride release, mass loss, and loss of sulfonic acid groups compared to pristine Nafion membranes, or membranes containing Zr02 or YSZ. (C) 2015 Elsevier B.V. All rights reserved.