The defect chemistry of polycrystalline has been studied by means of the equilibrium electrical conductivity as a function of temperature, oxygen activity, Sr/Ti ratio, and impurity additions. Reduction, excess , and acceptor impurities all contribute to the oxygen vacancy content and their effects are therefore highly interdependent. The effect of added donor‐impurities,e.g., 500 ppm Nb, is highly dependent on the presence and amount of excess .