The oxygen permeability and the CO2 tolerance of SrCoxFe0.9-xNb0.1O3-delta = 0.1-0.8) perovskite membranes were investigated by varying the composition of cobalt and iron in B-site of perovskite oxides. The experimental results show that the oxygen permeability increases while the CO2 tolerance decreases with increasing doping ratio of cobalt. The mechanism behind this trend was investigated by in situ high temperature XRD, FE-SEM, TGA, FTIR and XPS characterization techniques. These results indicated that basicity of SrCoxFe0.9-xNb0.1O3-delta is enhanced with increasing cobalt doping ratio because of a decrease in overall valence of B-site cations which leads to the decrease in electronegativity. Meanwhile, the decrease of valence for B-site cations also forms more oxygen vacancies which lead to the enhancement of oxygen permeability. With this trade-off between permeability and CO2 tolerance, it is necessary to balance oxygen permeability and stability under CO2 atmosphere for different applications. These results provide a guideline for the design of CO2 tolerant perovskite membrane. SCFN181 (SrCo0.1Fe0.8Nb0.1O3-delta) membrane was the most stable in the series of membranes studied. High oxygen permeation flux of 0.6 mL/min/cm(2) for 35 h at 900 degrees C and 0.26 mL/min/cm(2) flux at 800 degrees C for subsequent 100 h was achieved through SCFN 181 membrane (1.1 mm-thick) swept by CO2. With high permeability and high CO2-tolerant properties, SCFN181 membrane can meet the requirements for future industrial applications.