The dependence of the electrical conductivity, s, of Ba-2(2)5(In)O (on oxygen partial pressures, p)(, was estimated. Independent log s on log p)(, indicating oxide ion conduction, was observed at 850-1200C under log(p)(O2)(O2)(O2) (/ atm) of -0.7 to -3.0. Conduction by electrons generated from an oxide ion vacancy, the existence of which could be deduced by high-temperature X-ray diffraction (XRD), was detected in the brownmillerite phase below 900C under log(p)(O2) (/ atm) less than -3. The electrical conductivity under (logp)(O2) (/ atm) < -3 decreased according to the p)(O2) (decrease at a temperature range of 1000-1100C, suggesting that the vacancy decreased the oxide ion mobility in the distorted perovskite phase. At 1200C, a constant log s was observed in the log()(pO2) (/ atm) range of -0.7 to -4.0, indicating oxide ion conduction in the perovskite phase. The electrical conductivity of the specimen at more than 900C drastically decreased with the decrease of log(p)(O2) (/ atm) below -17, which can be attributed to the decomposition of Ba)(In)(O)(2)(2)(5) (revealed by high-temperature XRD under low log p)(o2). 2005 The Electrochemical Society. All rights reserved.