The thermal and chemical expansion properties of Ba0.5Sr05Co0.8Fe0.2O3-delta, a potential candidate for oxygen permeable membranes and cathodes in solid oxide fuel cells, were measured in the temperature range of 600 degrees C to 900 degrees C and the oxygen partial pressure (pO(2)) range of -4 to -0.67 log pO(2)/atm. The chemical expansion increased linearly with decreasing pO(2) and with increasing temperature, as interpreted by both the volume expansion due to the increased ionic radii of the B-site cations and the increased oxygen nonstoichiometry. The chemical diffusivity of oxygen and the surface exchange kinetics were successfully extracted from the transient chemical expansion relaxation profile for a small displacement from the equilibrium. The oxygen ionic conductivity was calculated from the Nernst-Einstein equation by combining the oxygen self-diffusivity from the chemical diffusivity and the thermodynamic enhancement factor from the oxygen nonstoichiometry data in the literature. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.017202jes] All rights reserved.