A detailed mathematical analysis is performed to understand the anode potential oscillations observed experimentally in a proton exchange membrane fuel cell (PEMFC) with H-2/CO feed (Ref. 9). Temperature and anode flow rate are found to be key bifurcation parameters. The time dependence of all the key surface species must be accounted for in order for the model to predict the oscillatory behavior, while the time dependence of CO concentration in the anode chamber need not necessarily be considered. The bifurcation diagram of CO electro-oxidation rate constant agrees very well with the effect of temperature on the oscillation pattern. The oscillator model is classified as a hidden negative differential resistance oscillator based on the dynamical response of the anodic current and surface species to a dynamic potential scan. A linear stability analysis indicates that the bifurcation experienced is a supercritical Hopf bifurcation. (C) 2004 The Electrochemical Society.