The effects of toluene contamination on the performance of polymer electrolyte membrane (PEM) fuel cells were investigated, using various levels of toluene concentration in the air streams, under different operational conditions and with different catalyst loadings. Constant-current polarization and electrochemical impedance spectroscopy (EIS) were conducted to analyze the poisoning behaviour of toluene. The severity of the contamination effect increased with an increase in both the current density and the toluene concentration, but decreased with an increase in both the relative humidity (RH) and the cathode-side Pt loading. The toluene-poisoned fuel cell could not be fully recovered by replacing toluene-contaminated air with pure air. EIS measurements revealed that both kinetic resistance and mass transfer resistance increased as a result of toluene contamination, while membrane resistance remained unchanged. However, the increase in kinetic resistance was a major contributor to cell performance degradation. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.