The temperature dependence of the electrical conductivity of Ba2In2O5 was investigated. The existence of three phases was observed, in agreement with the results of thermal analyses and high-temperature X-ray diffraction, which revealed that the crystal structure varied from brownmillerite (phase I) to distorted perovskite (phase II) to another distorted perovskite (phase III) with increase in temperature. Among the three phases, phase III showed the highest electrical conductivity and the lowest activation energy for oxide-ion transportation. The phase transition temperatures of the Ba2-xSrxIn2O5 and Ba2In2-yGayO5 systems were investigated to estimate the possibility of stabilizing phase III to low temperatures. It was clarified that the phase transition temperature decreased and that phase II disappeared as Ga content increased, that is, the tolerance factor approached 1. However, a decrease in the conductivity and an increase in the activation energy of phase III were observed with increase in Ga content. (C) 2004 Elsevier B.V. All rights reserved.