Electrochemical impedance spectroscopy (EIS) of the electrochemical oxidation of H-2 and H-2/CO mixtures on the proton exchange membrane fuel cell anode (Pt-Ru/C or Pt/C electrocatalysts) was performed at higher temperature (T-cell> 100 degrees C) and ambient pressure. Anode polarization studies were also conducted with the same H-2/CO mixture-fed streams. Both impedance and anode polarization provide information on the mechanism of electrochemical oxidation of H-2 and H-2/CO mixtures. The impedance spectra are presented and compared at various CO concentrations (0-500 ppm, 1 and 2%), cell temperatures (80-120 degrees C), relative humidity conditions (35-100%), as well as different dc bias potentials for characterizing the CO tolerance of the electrocatalysts. CO poisoning is alleviated by increasing the cell temperature, and relative humidity plays a strong role in the EIS and anode polarization. The impedance spectrum of the CO poisoned anode is strongly dependent on the dc bias potential. Under certain dc bias potential conditions, pseudoinductive behavior appears in the complex impedance plots, corresponding to the onset oxidation of CO as indicated from anode polarization. (c) 2005 The Electrochemical Society. All rights reserved.