X-ray diffraction (XRD), temperature programmed desorption (TPD), thermogravimetric analysis-differential thermal analysis (TGA/DTA) and oxygen permeation studies were combined to evaluate the oxygen behavior, order-disorder transition and phase stability of BaxSr1-xCo0.8Fe0.2O3-delta (x = 0-1.0) composite oxides. The X-ray diffraction patterns showed a cubic phase with perovskite structure for BaxSr1-xFe0.2Co0.8O3-delta (x <= 0.7) and also additional peaks for x > 0.7. Transition of the vacancy-ordered structure was observed in the TPD measurements for BaxSr1-xCo0.8Fe0.2O3-delta (x = 0.9 and 1) which confirmed by TGA/DTA. No ordering was observed in the Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF5582) system by any of the techniques utilized in this work. The combination of a high oxygen vacancy concentration and disordered structure leads to higher oxygen permeation fluxes through BSCF5582 membrane over the entire temperature region considered in this study. For example, the oxygen permeation flux was 2.25 cm(3) cm(-2) min(-1) at 950 degrees C. Results of the XRD after O-2-TPD showed that SrCo0.8Fe0.2O3-delta transformed to a non-perovskite structure after exposing to low P-O2 and high temperature. (C) 2011 Elsevier B.V. All rights reserved.