A mechanism that may cause accelerated performance decay of fuel cells is presented. The mechanism is explained using a one-dimensional model of the potential profile. The analysis indicates that the electrolyte potential drops from 0 to -0.59 V (vs. RHE) when the anode is partially exposed to hydrogen and partially exposed to oxygen. This causes flow of current opposite to normal fuel cell mode at the oxygen-exposed region and raises the cathode interfacial potential difference to 1.44 V, causing carbon corrosion, which decreases performance. The decay mechanism was validated using two different experimental setups which reproduced the carbon-corrosion phenomenon. © 2005 The Electrochemical Society.