The effects of trace concentrations of SO2 contaminant present in the cathode feed stream on proton exchange membrane fuel cell (PEMFC) performance are studied. Contaminant concentrations of 1, 2, and 10 ppm were exposed to the cell applying a total dosage of 160 mu mol of SO2 at 80 degrees C and a current density of 0.6 A cm(-2). All experiments show significant cell performance degradation before the steady-state poisoning state is reached. The performance degradation shows an inflection in the cell voltage, which is attributed to at least two different poisoning processes. The overall poisoning process is shown to consist of an irreversible part and a reversible part. While the performance loss of the reversible part is dependent on the SO2 concentration and is recoverable during a H-2/air operation, that of the irreversible part is greatly recoverable by potential cycling in the H-2/N-2 mode. Evidence is also presented that cathode exposure to SO2 results in a performance impact at the anode. Furthermore, sulfur species that remain in the membrane electrode assembly accelerate the cell performance degradation during a neat H-2/air operation and subsequent SO2 contaminant exposure. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3247546] All rights reserved.