High temperature X-ray diffraction (HT-XRD), temperature programmed desorption (TPD), thermogravimetric analysis-differential thermal analysis (TGA/DTA) and neutron diffraction were combined to determine the structure and oxygen stoichiometry of SrCo0.8Fe0.2O3-delta (SCF) and Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) up to 1273 K in the pO(2) range of 1 to 10(-5) atm. Formation of the vacancy-ordered brownmillerite phase, SrCo0.8Fe0.2O2.5, was observed as a region of zero oxygen release in the TPD measurements and confirmed by HT-XRD and TGA/DTA. No ordering was observed in the BSCF system by any of the techniques utilized in this work. The oxygen vacancy concentration of BSCF was found to be considerably higher than that of SCF and always higher than that of the ordered brownmillerite phase of SCF, delta=0.5. The combination of a high vacancy concentration and absence of ordering leads to higher oxygen permeation fluxes through BSCF membranes in comparison to SCE (c) 2006 Elsevier B.V. All rights reserved.