The oxygen nonstoichiometry of La0.6Sr0.4Co1-yFeyO3-delta (LSCF) mixed-conducting perovskite oxides strongly depends on the iron content: the cobalt-rich compounds (y = 0, 0.2, 0.4) were previously reported to show a sharp decrease in oxygen content with reducing atmosphere, while the oxygen vacancy concentration increased steadily for the iron-rich compounds (y = 0.6, 0.8) with a plateau at intermediate oxygen partial pressures for y = 1. Here we measured the oxygen content of LSCF with y = 0.5 by thermogravimetry and coulometric titration as a function of oxygen partial pressure and temperature. No transitional behavior could be observed for La0.6Sr0.4Co0.5Fe0.5O3-delta, the oxygen nonstoichiometry showed the same trend as for the cobalt-rich compounds. While the cobalt-rich compounds exhibit a metallic band structure, a semiconducting model fits the data best for the iron-rich compounds. Except for y = 1, the thermo-chemical stability of LSCF was little affected by the iron dopant content, all compounds (0 <= y <= 0.8) decomposed at similar oxygen partial pressures. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.050301jes] All rights reserved.