Mixed ionic and electronic conducting oxides SrFe1.5-xCoxOy (x = 0-0.6) were investigated. In the layered Sr-4(Fe,Co)(6)O13-delta, phase dominant region (x < 0.375), the average valence of the transition metals (Fe,Co) determined by iodometric titration decreases monotonically with increasing cobalt content and varies slightly with temperature. In the multiphase region (x. 0,375), the valence changes significantly with temperature. The electrical conductivity increases with increasing cobalt content. Unusual changes in electrical conductivity were observed at similar to 900 degreesC in the multiphase region, which may be associated with the conversion of the contained perovskite phase into the layered phase.