Strontium-doped lanthanum cobalt iron oxide is used in solid oxide fuel cells, oxygen separation membranes, and fuel reforming applications because of its mixed ionic and electronic conductivity. With changes in temperature and oxygen partial pressure (P-o2), the defect concentration changes and there is a corresponding change in strain (chemical expansion) and performance. In this paper, the oxygen nonstoichiometry of 0.5% A-site cation deficient La0.6Sr0.4Co0.2Fe0.8O3-delta is studied in the temperature range of 600-900 degrees C and modeled using a metallic model and three semiconductor models assuming itinerant electrons and holes or electrons localized on B-site. cations, with holes localized on either B-site cations or oxygen anions. The model consisting of holes localized on B-site cations fits the data best. The surface oxygen nonstoichiometry was determined by measuring oxygen loss of samples with different surface areas, and the surface is much more defective than the bulk. A long-term mass change measurement was also performed at 800 degrees C in air. and a mass loss was observed that may be due to cation segregation to the surface. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3194783] All rights reserved.