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A defect structure which incorporates singly and doubly ionized nonstoichiometric oxygen vacancies and acceptor states, presumably associated with impurities, was inferred from the agreement between the theoretically derived reIationand the experimentally observed 02-partial-pressure and temperature dependence of the Conductivity. This relation describes the behavior over the entire 0, pressure and temperature range investigated. A simplified defect model gave good agreement for the limiting case where n>>p but was shown to be in error compared with the more comprehensive relation above. The standard enthalpies of formation for singly and doubly ionized 0 vacancies in BaTiO, are 4.3 and 5.1 eV, respectively. The difference between these values, 1.4 eV, corresponds to a donor energy level which lies near the middle of the band gap.