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The photorefractive Properties of a nominally pure single crystal of BaTiO3 were altered by treating the crystal at 650 °C in oxygen at different partial pressures. Treatment altered the effective density of photorefractive charge carriers in the crystal and couls convert an inactive crystal into an active one. Treatment at low oxygen pressure (reduction) decreased the temperature of the tetragonal-to-cubic phase transition of the crystal and also decreased the measured optical band gap, implying that oxygen vacancies had been introduced into the bulk crystal. These oxygen vacancies are associated with negative photorefractive charge sonors. Either hole transport or electron transport dominated, depending on whether the partial pressure of oxygen was greater than or less than 1/2 atmosphere during treatment. The competing roles of electrons and holes are discussed.