A defect chemistry model is proposed for the brownmillerite oxides with high oxygen ion conductivity. Ba2In2O5 was chosen as the model material and its electrical conductivity and transport properties have been studied in detail. The oxygen ion conduction above the order-disorder temperature, T-d approximate to 925 degrees C, and the mixed ionic-electronic conduction below T-d, was studied by conductivity and EMF measurements as a function of temperature and oxygen activity. The main defects are intrinsic anion Frenkel defects below T-d, and above T-d the oxide can be treated as acceptor-doped perovskite with extrinsic oxygen vacancies. Charge compensation involves only ionic defects over the whole P-O2 range used in this study. The formation and mobility enthalpies of the Frenkel defects, the redox enthalpies, and the band gap have been obtained for this oxide. The proposed model is in good agreement with the experimental results. Decomposition of Ba2In2O5 with reduction of In3+ is indicated by the conductivity measurements. Degradation in CO2 atmospheres with formation of BaCO3 and volatilization of In2O3 was also observed by XRD and EDS techniques. It is clear that In3+ will have to be replaced by an element with a more stable 3 + oxidation state (e.g. Y, Er, Ga), if a brownmillerite compound is to be a useful solid electrolyte over a wide range of oxygen activities.