Toluene, one of many volatile organic compounds existing in the atmosphere, was studied as an impurity or contaminant in the airstream of proton exchange membrane (PEM) fuel cells. The effects of toluene contamination on fuel cell performance were investigated with constant-current polarization discharge under various operating conditions, including current density, back pressure, and air stoichiometry. The severity of the contamination increased with increasing current density, toluene concentration, and air stoichiometry, but decreased with increasing back pressure. The effect of toluene contamination on current dynamic operation was also studied by cycling the current density from 0.0 to 0.2 A cm(-2) and then to 1.0 A cm(-2) in the presence of toluene. Cell performance degradation during current dynamic operation was accelerated by the presence of toluene. Electrochemical impedance measurements and analysis revealed that both kinetic resistance and mass transfer resistance were increased as a result of toluene contamination, but the increase in kinetic resistance was dominant.