Perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) is considered as one of most promising materials for the oxygen separation from air. In order to assess the impact of asymmetric architecture on oxygen transport properties and stability, dense ceramics and membranes consisting of two porous and one dense layers were fabricated and tested. The stability of Ba0.5Sr0.5Co0.8Fe0.2O3-delta in atmospheres with low oxygen partial pressure is similar to that for SrCo0.8Fe0.2O3-delta. Analysis of the thickness dependence of oxygen permeation fluxes through symmetric dense membranes unambiguously showed that the permeation is limited by surface exchange kinetics at d <= 1 mm. The main limitations for the oxygen permeation through asymmetric membranes are associated with gas diffusion in pores and oxygen desorption process, despite a high open porosity (49%) of the porous layers. The asymmetric membranes demonstrated a stable performance if using inert sweep gas. On exposing to CO2/H2O containing atmospheres, the permeation decreases dramatically due to a deep decomposition process on the permeate side, resulting in the formation of carbonates and/or hydroxides causing blocking of oxygen transport. (C) 2010 Elsevier B.V. All rights reserved.