AC impedance spectra of symmetrical cells consisting of Pt/Ce0.9Gd0.1O1.95-delta/Pt have been measured in air and under different reducing atmospheres at temperatures in the range 250-650 degreesC. The electrode response under reducing conditions shows a contribution extending to very low frequencies and thus a large apparent capacitance. This is caused by chemical diffusion in the electrolyte, which becomes a mixed ionic-electronic conductor under reducing conditions. The spectra are interpreted quantitatively using a theory developed by Jamnik and Maier and literature data for the nonstoichiometry, chemical diffusion, and ionic and electronic conductivity of the electrolyte. The results show that misleading conclusions can be drawn concerning the electrochemical properties of the electrodes (low charge-transfer resistance, high capacitance and gas-transport limitations! under reducing conditions if the conventional equivalent circuit for a purely ionic electrolyte is used instead of the proper analysis. The results also delineate the conditions that must be satisfied in order for useful information about the electrode to be extracted from the impedance spectrum. (C) 2004 The Electrochemical Society.