Germanium and iron co-doped SrCoO2.5 + (delta) was investigated in terms of phase stability, oxygen permeability and electrical conductivity. The favorable high-temperature cubic structure of SrCoO2.5 + delta was stabilized to lower temperatures by co-doping Ge (10 mol%) and Fe (10 mol%) that substituted for Co, which however could not be achieved by doping Ge (20 mol%) alone. In contrast to SrCo0.8Ge0.2O2.5 (+ delta) sample which showed a sharp decrease in oxygen permeability at temperature of similar to875 degreesC upon cooling, SrGe0.1Co0.8Fe0.1O3-delta sample remained well-permeable to oxygen at lower temperatures down to at least 820 degreesC; an abrupt change in electrical conductivity in SrCo0.8Ge0.2O2.5 (+ delta) also occurred accompanying the phase transition. The oxygen permeation flux for SrGe0.1Co0.8Fe0.1O3 - delta increased significantly with the decrease of the membrane thickness, indicating the transport of oxygen ions in the bulk of the membrane as the rate-limiting step. (C) 2004 Elsevier B.V. All rights reserved.