Previous tests revealed that SO2 contamination in proton exchange membrane fuel cells (PEMFCs) resulted in a two-stage degradation in cell voltage. Unfortunately, the effect of SO2 on the cell cathode reactions and the corresponding SO2 contamination mechanism are still unknown. In this paper, the effect of SO2 contamination on oxygen reduction reactions (ORR) was analyzed using electrochemical impedance spectroscopy (EIS) through an equivalent electrical circuit model (ECM). The ORR parameters extracted from the ECM suggested that during the first poisoning stage, the SO2 adsorption increased the charge transfer resistance of the ORR, causing the initial cell performance degradation. During the second poisoning stage, SO2 adsorption may assist in shifting the ORR from the "direct four electron pathway" to the "series two electron pathway." This shift resulted in a decrease in the current efficiency of the ORR, which then resulted in the subsequent degradation of cell performance. The increase in the fluorine emission rate, as detected by ion chromatography, was indirect evidence for the ORR pathway shift during the second poisoning stage and the decrease in current efficiency. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.067205jes] All rights reserved.