An investigation was conducted to determine and compare the effect of cell current pulsing and "self-oxidation" in increasing the CO tolerance of a PEM fuel cell. The most effective pulsing parameter values were also determined. Current pulsing involves periodically demanding positive increases in current from the fuel cell to create an anode over-potential that oxidizes CO from the catalyst surface, while "self-oxidation" or sustained potential oscillations is achieved when the anode catalyst becomes so saturated with CO that the anode over-potential increases to a value at which CO is oxidized from the catalyst. The CO tolerance of a fuel cell system with a Pt-Ru anode was tested using 50 and 496 ppm CO in the anode fuel. The performance of the system declined with an increase in CO concentration. Current pulses of various amplitude, frequency, and duty cycle were applied to the cell while CO was present in the anode fuel. With 496 ppm CO in the anode fuel, a pulse of 1.0 A/cm(2), 0.5 Hz, and a 20% duty cycle proved most effective. When the cell was exposed to approximately 500 ppm CO, without employing pulsing, "self-oxidation" occurred and CO was periodically oxidized from the catalyst surface. However, pulsing allowed the cell to operate at a constant voltage and power a higher percentage of the time than "self-oxidation"; hence, pulsing was more effective in increasing the CO tolerance of the cell. (C) 2004 Elsevier B.V. All rights reserved.