The influence Of CO2 on the oxygen permeation performance of perovskite-type (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-delta (BSCF) membranes under different experimental conditions is presented. First, pure CO2 was applied as the sweep gas at 875 degrees C yielding an immediate cessation of the oxygen permeation. In order to probe the reversibility of this stagnancy, several cycles of changing the sweep gas between helium and CO2 were conducted. The analysis of the microstructure after permeation experiments were carried out by transmission electron microscopy (TEM), X-ray diffraction (XRD) as well as by scanning electron microscopy (SEM). It was found that both microstructure as well as oxygen permeation are recovered in a helium atmosphere. Additionally, long-time treatment with pure CO2 for 72 It on the permeate side and the concentration effect of CO2 in the sweep gas were accomplished showing that the perovskite structure is impaired only up to a depth of 50 mu m. Further on, the impact of CO2, on the feed side was examined by adding certain amounts Of CO2 to the feed air. It was found that the impact Of CO2 on the oxygen permeation applied in the sweep gas is superior to that applied on the feed gas. (c) 2007 Elsevier B.V. All rights reserved.