Sustained potential oscillations are experimentally observed in a proton exchange membrane fuel cell with PtRu as anode catalyst and with H-2/108 ppm CO as the anode feed when operating under a constant current density mode. These oscillations appear at fuel-cell temperatures below 70degreesC. A threshold value exists for both the current density and the anode flow rate at a given fuel-cell temperature for their onset. The temperature dependence of the oscillation period shows an apparent activation energy around 60 kJ/mol. The potential oscillations are believed to be due to the coupling of anode electro-oxidation of H-2 and CO on the PtRu catalyst surface, on which OHad is formed more readily, i.e., at lower overpotentials. A simple kinetic model is provided that can reproduce the observed oscillatory phenomenon both qualitatively and quantitatively.