The major source of membrane chemical degradation during proton exchange membrane fuel cell operation is from attack of radicals, which form when hydrogen and oxygen react on platinum. It is hypothesized that the higher durability of PtCo/C electrodes will decrease platinum deposition in the membrane and, therefore, increase membrane durability. After testing catalyst coated membranes (CCMs) containing either Pt/C or PtCo/C under open circuit voltage, it was observed that using PtCo/C rather than Pt/C resulted in lower CCM degradation in terms of voltage decay, fluoride emission, and performance loss before and after testing. Transmission electron microscopy images showed that platinum with particle sizes of 10 to 35 nm were dispersed throughout the membrane of the CCM containing PtCo/C, while the CCM containing Pt/C resulted in formation of a 2.5 micron wide platinum band 2.3 microns from the cathode with particle sizes of 15 to 85 nm. The increased degradation observed for the cell containing Pt/C compared to the cell containing PtCo/C is attributed to the formation of the Pt band in the cell containing Pt/C. Reactant crossover is stoichiometrically favorable for OH radical generation at the location of the Pt band, resulting in localized radical attack of the polymeric membrane. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.094205jes] All rights reserved.