As one of the most deleterious impurities to proton exchange membrane fuel cells (PEMFCs), sulfur dioxide (SO2) in air can pass through the membrane from the cathode to the anode and poison the catalyst of the two electrodes. The phenomenon of SO2 crossover is investigated electrochemically in this paper. The influences of SO2 concentration, relative humidity, gas pressure and current density on SO2 crossover are discussed. Experimental results reveal that the anode tends to be poisoned heavily with the increasing concentration of SO2 in the cathode. The coverage of the anode catalyst by SO2 permeating from the cathode enlarges with the decreasing relative humidity in the anode. The rate of SO2 crossover from the anode to the cathode is promoted at high current density when SO2 is directly introduced into the anode side instead of the cathode side, which can be ascribed to the electro-osmotic drag effect. Gas pressures show no obvious effects on SO2 crossover. A co-permeation mechanism of SO2 with water is deduced based on the overall analysis. (c) 2010 Elsevier B.V. All rights reserved.