The effects of NOx on the performance of proton exchange membrane (PEM) fuel cell were investigated through the introduction of a mixture containinL, NO and NO2, in a ratio of 9:1, into the cathode stream of a single PEM fuel cell. The NOx concentrations used in the experiments were 1480 ppm, 140 ppm and 10 ppm. which cover a range of three orders. The experimental results obtained from the tests of durability, polarization, reversibility and electrochemical impedance spectroscopy (EIS) showed a detrimental effect of NOx on the cell performance. The electrochemical measurements results suggested that the impacts of NO, are mainly resulted from the superposition of the oxygen reduction reaction (ORR), NO and HNO2 oxidation reactions. and the increased cathodic impedance. Complete recovery of the cell performance was reached after operating the cell with clean air and then purging with N-2 for hours. (c) 2005 Elsevier Ltd. All rights reserved.