Measurements of the direct current-voltage characteristics of calcia-stabilized zirconia using platinum electrodes of various types have been made to investigate possible electrode processes. Depending on the form of the electrode, the rate-limiting step in the electrical conduction sequence can be (a) ionic conduction by O = in the zirconia, (b) electronic conduction in the zirconia, (c) oxygen atom diffusion through the platinum cathode, (d) the transfer of oxygen from the gas phase into the zirconia at the zirconia-gas interface on the cathode side. The conditions, under which each of these situations exists, are explained in terms of the structure of the electrode.Earlier studies on cells Pt, 02[0.85 ZrO2 9 0.15 CaOIPt, 02 with platinum point electrodes (1), or porous platinum electrodes (2), showed the I-V characteristic to be markedly nonohmic. The rate-controlling step responsible for this behavior was found to be confined to the cathode. Two mechanisms of electrode operation appeared to be important (2). At low voltages (e.g., * Electrochemical Society Active Member.