The electrochemical oxidation of H-2 and CO in a H-2-H2O-CO-CO2 system at the interface of a porous Ni-yttria stabilized zirconia (YSZ) cermet electrode and YSZ electrolyte has been studied using complex-impedance spectroscopy and direct-current polarization measurements of 1073 and 1273 K under a constant oxygen partial pressure. The polarization resistance increased when the CO concentration ratio, p(CO)/[p(H2) + p(CO)], exceeded 0.2 and 0.5 at 1023 and 1273 K, respectively. The electrochemical oxidation rate of H-2 was 1.9-2.3 times and 2.3-3.1 times higher than that of CO at 1013 and 1273 K, respectively, and the water-gas shift reaction was found to be much faster than the electrode reaction at both temperatures. An equivalent-circuit analysis of the complex-impedance spectra suggested that the lower electrochemical oxidation rate of CO (compared to H-2) was caused mainly by the larger diffusion resistance of CO than H-2 on the electrode surface at 1023 K, and bg both the larger surface diffusion resistance and charge-transfer resistance at 1273 K.