Dense dual-phase membranes 40% Sm0.5Sr0.5Cu0.2Fe0.8O3-delta-60% Ce0.8Sm0.2O2-delta (SmSrCF5528-CSO) and 40% Sm(0.3)Sro(7)Cuo geo(0.3)- 60% Ce(0.8)Smo(2)O(2-delta) (SmSrCF3728-CSO) were prepared via the citric acid/EDTA complexing sol-gel process. The phase composition and the microstructure of the membranes were characterized by XRD, SEM and EDXS analysis. The newly developed cobalt-free membranes SmSrCF5528-CSO and SmSrCF3728-CSO (0.6 mm thick) exhibited high oxygen fluxes of 1.12 and 1.15 cm3 min(-1) cm(-2), respectively, at 1000 degrees C with pure CO2 as the sweep gas. The long-term CO2 stability of each membrane was studied in the temperature range of 900-1000 degrees C for more than 200 h on stream. The SmSrCF5528-CSO membrane displayed very good stability at all temperatures. The oxygen permeation fluxes of the Sr-rich membrane SmSrCF3728-CSO were stable at 1000 and 950 degrees C. (C) 2016 Elsevier B.V. All rights reserved.