A solid-state reaction was used to prepare Sr1-x(Fe0.5Co0.5)O3-delta (x = 0-0.3) mixed (electronic and ionic)conducting oxides. An A-site cation deficiency in Sr1-x(Fe0.5Co0.5)O3-delta (x = 0.05-0.1) causes the Sr-rich phase of grain and grain boundaries to disappear. It also enhances the stability of the perovskite structure and reduces thermal expansion at high temperatures in an Ar atmosphere. However, a further deficiency of Sr causes the segregation of a considerable amount of Co3O4 and FeO. As the Sr deficiency is increased, there is a slight decrease in the electrical conductivity in air and improvement of the oxygen permeation fluxes of slightly Sr-deficient membranes (x = 0.05, 0.1). The effect of Sr deficiency on the oxygen transport in the Sr1-x(Fe0.5Co0.5)O3-delta membrane is discussed in terms of the microstructure and phase transition. (C) 2012 Elsevier B.V. All rights reserved.