Defect perovskite-type BaFeO2.5 was oxidized to BaFeO3 by an electrochemical method in an alkaline aqueous electrolyte at 25 degrees C. With increase in the charge transfer q (electrons/formula unit), BaFeOz showed monoclinic (at nearly q=0), rhombohedral (at around q=0.34), pseudo-cubic (at 0.78 <= q <= 0.89), and true cubic (at q >= 0.89) structures. While an oxygen evolution reaction (OER) started at ca. q=0.80, the oxidation of BaFeOz, could concurrently be continued with the OER and BaFeO3 was obtained at q=1.2. Further, the chemical potential of oxygen in BaFeOz, could be estimated in the range of 2.5 <= z <= 2.87 without OER (C) 2014 Elsevier Ltd. All rights reserved.