The electrochemical oxidation of H2/CO mixtures (100 ppm and 2% 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 or diffusion), while a high-frequency arc provides evidence for a more rapid processthe charge transfer on holes preexisting in the CO ads adlayer; (b) a state between 0.3 V and a critical potential (Vcrit), 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 Vcrit, in which the diameters of the two loops in the EIS become equal. Vcrit may be used as a diagnostic criterion for CO tolerance, with a significance close to that of the ignition potential. The differences in Vcrit with respect to the values obtained on Pt-based GDE are assigned to the increase of the ratio between the rate of COads oxidation (vox) and the rate of CO adsorption (vads). Stripping voltammetry and polarization curves, recorded in situ, are used to support the conclusions obtained from impedance measurements.