Defect chemistry for a mixed conductor, La0.6Sr0.4Co0.2Fe0.8O3-delta was studied. Samples were treated under controlled oxygen partial pressure, P(O-2), conditions at 1273 K [10(-11.1) <= P(O-2)/atm <= 1], and cooled to room temperature. Oxygen nonstoichiometry and valences of transition metal ions for the treated samples were evaluated by iodometry and X-ray absorption spectroscopy, respectively. With decreasing P(O-2), preferential reduction of Co3+ to Co2+ was observed, while iron preserved its higher valence above 3 under conditions studied. A dependency of its electrical conductivity on P(O-2) was discussed along with a change in concentration of oxygen vacancies and mixed valences. (c) 2006 Elsevier B.V. All rights reserved.