The performance of perovskite-type oxygen-transporting membranes during CO2 sweeping has been investigated. The series of La1-xSr1-xCo0.8Fe0.2O3-delta (LSCE) materials with x=1. 0.8, 0.6 and 0.4 (SCF, LSCF2882, LSCF4682, LSCF6482) were synthesized by the sol-gel method and characterized by X-ray diffraction (XRD). To investigate the poisoning effect of CO2 on the phase composition and microstructure of the membranes after long-term oxygen permeation experiments (more than 200 h) at 1173 K under a CO2 atmosphere, transmission electron microscopy (TEM) investigations were carried out. Electron-energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDXS) revealed the formation of carbonate at the outer surface of the LSCF materials with low lanthanum content, which correlates with a decrease in oxygen permeation flux. Nevertheless, the LSCF 6482 material is tolerant to CO2 gas. No carbonate layer was found after 200 h of operation, and the oxygen flux remained stable. (C) 2015 Elsevier B.V. All rights reserved