The influence of carbon monoxide poisoning on the platinum and platinum-ruthenium anode in a polymer electrolyte fuel cell was investigated using electrochemical impedance spectroscopy (EIS). EIS is a very useful method for the characterisation of fuel cells. Therefore, impedance measurements of the cell under constant load were performed at periodic time intervals. Due to the poisoning effect of the carbon monoxide, the system changes its state during the experiment. The reconstruction of quasi-causal spectra was made possible using enhanced numerical procedures, especially the time course interpolation and the Z-HIT refinement. The reconstructed impedance spectra show a strong time dependence and exhibit pseudo-inductive contributions at the low-frequency part of the spectra which increase during the experiment. The analysis of the spectra suggests that the pseudo-inductive behaviour can be attributed to a surface relaxation process of the anode. Furthermore, the influence of the carbon monoxide on the electrochemical behaviour of the contaminated fuel cell may be interpreted by means of a Faraday impedance in addition to a potential-dependent hindrance of the charge transfer. (C) 2003 Elsevier Ltd. All rights reserved.