The oxygen stoichiometry of Ba0.5Sr0.5Co0.8Fe0.2O3 (-) (delta) (BSCF) was determined by isothermal thermogravimetry between 500 and 1000 degrees C in combination with a redox titration. The oxygen stoichiometry of BSCF varies from 2.47 at log p(O2) = 0 to 2.36 at log p(O2) = -5.6 at 700 degrees C and from 2.42 at log p(O2) = 0 to 2.25 at log p(O2) = -5.6 at 1000 degrees C, respectively. At T = 500 degrees C partial decomposition of BSCF is observed. The variation of the oxygen stoichiometry as a function of oxygen partial pressure between 700 <= T <= 1000 degrees C is described with a semi-empirical defect model. The thermal expansion of BSCF and the macroscopic chemical expansion were determined for different oxygen stoichiometries by dilatometry in an inert atmosphere. The microscopic chemical expansion, which was derived from the variation of lattice constants of samples synthesized at low oxygen partial pressures, agrees well with the macroscopic chemical expansion. The chemical expansion coefficient of BSCF is smaller compared to the majority of mixed conductors. Moreover, the variation of the thermal expansion coefficient with the oxygen stoichiometry is small. (C) 2009 Elsevier B.V. All rights reserved.