The CO, O2, PtAu/ZrO2(0.92)Y203(0.04)YbO3(0.04)/Pt, air system has been recently proposed as CO, O2 sensor. Thermodynamic measurements of the open-circuit potential, E0, of the interphase CO, O2, PtAu/ZrO2(0.92)Y2O3(0.04)Yb2O3(0.04) as a function of the carbon monoxide/oxygen pressure ratio were carried out at T = 773 K and T = 873 K and confirmed the recently obtained results that the cell operates as a reproducible and CO-sensitive sensor. To clarify whether the carbon monoxide takes part in an electrochemical reaction step or is oxidized only in a chemical step, i.e. in a heterogeneous catalytic reaction, dynamic measurements using electrochemical impedance spectroscopy (EIS) were per-formed. They show a mixed charge transfer and transport controlled kinetic behavior. The polarization resistance, R(p), decreases with increasing P(CO)/P(O2) ratio at constant P(O2). This clearly indicates an electrochemical step of carbon monoxide oxidation in the overall reaction in agreement with previous results obtained for pure platinum paint electrodes.