The phase stability of SrCo0.8Fe0.2O3-d perovskite doped with niobium was studied by in situ high-temperature X-ray diffraction in the temperature range of 30-1000 degrees C and oxygen partial pressure 0.2-10(-5) atm. The stability of the cubic perovskite structure in a wide range of oxygen partial pressures is the main advantage of SrCo0.8-xFe0.2NbxO3-d (x = 0.1-0.3) system in comparison with SrCo0.8Fe0.2O3-d. It is suggested I hat equilibrium of the thermal expansion with changes of the oxygen non-stoichiometry leading to the same lattice parameters in the oxidizing and reducing environments at the catalytic temperatures is a necessary requirement for stable operation of perovskite as an oxygen-conducting membrane. In the case of SrCo0.8-xFe0.2NbxO3-d perovskite this condition is met at x = 0.2. This makes the SrCo0.6Fe0.2Nb0.2O3-d composition promising for application as oxygen-conducting membrane. (C) 2009 Elsevier B.V. All rights reserved.