The d‐c polarization technique according to Wagner was applied for the first time at elevated temperatures for the determination of σ ⊖ and σ ⊕ in the Zr0.85Ca0.15O1.85 and Th0.85Y0.15O1.925 electrolytes over a range of oxygen activity and temperature. The applicability of the technique and an improved method for data analysis are demonstrated. Experimental difficulties and limitations are discussed. The total conductivity of Zr0.85Ca0.15O1.85 was determined over a range of oxygen activity and temperature.