In situ EIS data are presented on the anodic process in proton exchange membrane (PEM) fuel cells and the influence of CO poisoning of the Pt gas-diffusion electrodes (GDE) is examined. A characterization of the effects of interfacial kinetics in determining polarization losses in the presence of CO is performed by comparing impedance patterns obtained for cells of the type H-2/H-2 with H-2/(H-2 + CO). The impedance spectrum of the poisoned electrode is strongly dependent on potential and on the CO concentration in the gas: feed. In the range between 0 and 0.3 V the impedance increases rapidly with the potential, while at potentials higher than 0.3 V a "pseudoinductive" behavior is observed, in which a positive loop at higher frequencies is accompanied by a low frequency (LF) loop in the fourth quadrant. The latter was assigned to a new rate-determining process, the oxidation of COads by Pt-H2Oads. As a critical potential V-crit is attained, the diameters of the two loops become almost equal and the LF limit of the impedance (R-0) approaches the value for unpoisoned electrode, showing that the activity of the electrode activity has been restored. The value of V-crit is 0.43 and 0.58 V for electrodes poisoned with 100 ppm and 2% CO, respectively. Ar very high potentials, where the oxidizing species are Pt-OHads, the impedance pattern is reversed to the second and third quadrants. Stripping voltammetry and polarization curves recorded ill situ, are used to support the conclusions obtained from impedance measurements.