Analysis of the gas, Pt/yttria‐stabilized zirconia electrode shows that electrocatalyzed reactions can occur at both the anode and the cathode. These reactions are promoted by the onset of electronic conductivity in the electrolyte subsurface. Such conductivity can result from either native or added electronic defects, which become active under appropriate experimental conditions pertaining to oxygen partial pressure or electrode polarization. Transition from pure ionic to mixed (ionic + electronic) subsurface conductivity is reflected by a sharp decrease in the corresponding overvoltage and by a drastic modification of the electrode impedance spectra in terms of magnitude and frequency distribution. Inductive loops appear to be characteristic of this transition. The promoting role of electronic defects in the electrode process is clearly demonstrated. At both the cathode and the anode, the contribution of mobile electronic defects also results in a spreading of the reaction zone around the triple contact point.

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