The phase stability, oxygen stoichiometry and expansion properties of SrCo0.8Fe0.2O3-delta (SCF) were determined by in situ neutron diffraction between 873 and 1173 K and oxygen partial pressures of 5 x 10(-4) to 1 atm. At a pO(2) of 1 atm, SCF adopts a cubic perovskite structure, space group Pm (3) over barm, across the whole temperature range investigated. At a pO(2) of 10(-1) atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr2Co1.6Fe0.4O5, Space group Icmm. A pure brownmillerite phase is present at pO(2) of 10(-2) and 5 x 10(-4) atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a two-phase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO6 (B=Co, Fe) octahedra is present in the brownmillerite structure with apical bond lengths of 2.2974(4)angstrom and equatorial bond lengths of 1.9737(3)angstrom at 1021 K and a pO(2) of 10(-2) atm. SCF is highly oxygen deficient with a maximum oxygen stoichiometry, 3-delta, measured in this study of 2.58(2) at 873 K and a pO(2) of 1 atm and a minimum of 2.33(2) at 1173 K and a pO(2) of 5 x 10(-4) atm. Significant differences in lattice volume and expansion behavior between the brownmillerite and cubic perovskite phases suggest potential difficulties in thermal cycling of SrCo0.8Fe0.2O3-delta membranes. (c) 2006 Elsevier B.V. All rights reserved.