PEM fuel cell with oxygen on the anode side splits into the direct (D) and reverse (R) domains. In the R-domain, carbon in the cathode catalyst layer is utilized as a fuel, while oxygen on the anode side is reduced. A model of the process based on (C.A. Reiser et al. ESSL, 8 (2005) A273) shows that the carbon corrosion current is in the order of the oxygen-limiting current density in the cell. The model reveals formation of a local hydrogen-oxygen fuel cell at the D/R interface, which supports large in-plane proton current in the membrane. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596169] All rights reserved.