The electrochemical oxidation of H-2/CO mixtures (100 ppm and 2 percent CO) on Pt/Ru gas diffusion electrodes (GDE) is examined using in-situ electrochemical impedance spectroscopy (EIS). The impedance spectrum of the poisoned electrode is strongly concentration, and potential dependent, showing evidence for three states of activity of the surface: a) a state in the 0-0.3 V potential range, in which the impedance is very high and increases with the bias potential; a low-frequency loop in EIS is assigned to the rate-determining step (CO adsorption of diffusion), while a high-frequency arc provides evidence for a more rapid process-the charge transfer on holes preexisting in the CO (ads) adlayer; (b) a state between 0.3V and a critical potential (V-crit), in which the impedance decreases with potential: the characteristic feature in this region is the appearance of a pseudo-inductive pattern, with a negative (inductive) loop at the lowest frequencies, assigned to the oxidation of Pt-COads by Ru-OHads. The time scale of this process may be evaluated form EIS, which enables a discussion of the effects of CO concentration, potential, and temperature; (c) a state at potentials higher than V-crit, in which the diameters of the two loops in the EIS become equal. V-crit may be used as a diagnostic criterion for CO tolerance, with a significance close to that of the ignition potential. The differences in V-crit with respect to the values obtained on Pt-based GDE are assigned to the increase of the ration between the rate of COads oxidation (v(ox)) and the rate of CO adsorption (v(ads)). Stripping voltammetry and polarization curves, recorded in situ, are used to support the conclusions obtained from impedance measurements.