The oxygen permeation flux through Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) membranes of different thicknesses (1.5-2.5 mm) was measured between 800 and 900 degrees C under vacuum conditions. The vacuum pressure at the permeate side of the membranes was varied from 5 mbar to 30 mbar. In order to examine the effect of membrane configuration, an asymmetric BSCF membrane with a thin-film layered structure was produced. A significant improvement in oxygen permeability was observed in the asymmetric membrane, which showed an oxygen flux of 32.45 ml min(-1) cm(-2) [STP] at 900 degrees C, much higher than that of dense membranes. The long-term permeation study showed that the membrane properties of the BSCF material undergo gradual deterioration at 800 degrees C due to phase decomposition. The hexagonal phase formed primarily in grain boundaries, and after the saturation of boundary nucleation sites, phase transition also occurred within the matrix. The SEM micrographs of BSCF membranes operating over long periods exhibit more rapid deterioration at the air feed side than at the oxygen-lean side. (C) 2013 Elsevier B.V. All rights reserved.