BaxSr1-xCo0.8Fe0.2O3-delta (x = 0 similar to 1.0) composite oxides were prepared by a combined EDTA and citrate complexing method. The doping effect of Ba on the phase structure, phase stability and oxygen permeation properties of the mixed conductors was investigated with combined XRD, H-2-TPR, O-2-TPD techniques and oxygen permeation studies. It was found that the perovskite structure was sustained in the whole range of the oxygen activity investigated (0.21 similar to 10(-6) atm) when 0.3 less than or equal to x less than or equal to 0.5. But for the materials with a differed doping content of Ba (x = 0 similar to 0.1, 0.7 similar to 1.0), a phase transition was accompanied which was deleterious to the integrality of the membrane during operation. The relationship of phase structure and stability with tolerance factor was proposed. The doping of Ba in Sr(Co0.8Fe0.2)O3-delta could not lead to the improvement of these materials in the resisting of reduction, but the synergetic effect between cobalt and iron in the material was improved. Considerable high oxygen permeation rates were found for the BSCFO membranes in the whole range of Ba doping content. The Ba0.3Sr0.7Co0.8Fe0.2O3-delta membrane had the highest oxygen permeation flux, (1.19 ml/cm(2) min for 1.50 mm thickness membrane at 850 degreesC), the lowest activation energy for oxygen transportation (37.8 KJ/mol at the temperature range of 731 degreesC similar to 950 degreesC), and the lowest critical temperature for the change in the activation energy (731 degreesC).