Sulfur dioxide (SO_2) has an effect on degradation of catalyst layers and/or membrane in polymer electrolyte membrane fuel cells (PEFCs). In this study, temporal variation of cell voltage under open circuit condition was monitored with various SO_2 concentration and humidification. Cell voltage drop and its relaxation time were well correlated with SO_2 concentration. Pt utilization rate characterized by cyclic voltmmograms were also carried out to investigate poisoning behavior by SO_2. It was observed that SO_2 contamination in the anode was intensified at lower relative humidity condition.