The work deals with the studies of Ba0.5Sr0.5Co0.8-xWxFe0.2O3-delta (x=0-0.1) materials obtained by the partial substitution of cobalt in Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) by tungsten. It is shown by means of XRD that the solubility limit of tungsten in the structure of cubic perovskite BSCF is a bout 2% < x < 3%. Continuous equilibrium << 3 - delta - lg pO(2) - T >> diagrams obtained by oxygen release technique are presented for the compositions BSCF and Ba0.5Sr0.5Co0.78W0.02Fe0.2O3-delta (BSCFW2) for the first time. The diagrams allowed us to detect the new phase transition P-1 (low temperature phase, Pm (3) over barm) - P-2 (high-temperature phase, Fm (3) over barc). It is demonstrated that Co substitution by highly charged cation W+6 (2%) improves the functional properties of BSCF; increases the oxygen permeability, suppresses phase transition << cubic-hexagonal >> perovskite, enhances the stability of membrane material in the atmosphere of CO2, which provides the long-term stability of oxygen fluxes. The dependence of oxygen fluxes on membrane thickness provides evidence that for L >= 1.60 mm oxygen permeability is limited by the bulk diffusion of oxygen ions. (C) 2014 Elsevier B.V. All rights reserved.