The attack by HO center dot on the ionomer in the polymer electrolyte fuel cell (PEFC) can be mitigated by the incorporation of regenerative radical scavengers, such as cerium and manganese, into the membrane. The influence of the presence of the Ce and Mn ions on the reaction of HO center dot with the perfluoroalkylsulfonic acid (PFSA) ionomer was studied in the framework of a kinetic model. Scavenging of HO center dot by Ce3+ and Mn2+ leads to a decrease in the steady-state concentration of HO center dot and, therefore, the rate of ionomer attack, where Ce is more effective than Mn. The HO center dot scavenging yield was found to decrease with increasing reactive end-group concentration of the PFSA ionomer. The oxidized metal ions are rapidly reduced by both and HOO center dot, which restores the scavenger and ensures that the majority (> 99.99%) of metal ions is in the reduced, "HO center dot-scavenging-active" oxidation state. This stabilization mechanism is not expected to be operational in hydrocarbon membranes, because HO center dot reacts with aromatic units far too rapidly. Also, the effectiveness of the mitigation mechanism is limited under the conditions of an ex situ Fenton test due to the abundance of H2O2, which is confirmed by experimental data reported in the literature. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.075202jes] All rights reserved.