The oxygen nonstoichiometry of La0.6Sr0.4Co1-yFeyO3-δ (y = 0.2, 0.4, 0.6, 0.8) was measured by high temperature gravimetry and coulometric titration. The results of the oxygen nonstoichiometry measurements could be categorized into Co rich compounds (y = 0.2, 0.4) and Fe rich compounds (y = 0.6, 0.8). The Co rich compounds decomposed into A2BO4 and CoO at relatively high PO2, i.e. ca.10−9 bar at 873 K, and ca. 10−6 bar at 1073 K. The apparent decomposition PO2 in the case of the Fe rich compounds shifted to the lower PO2 side, i.e. ca.10−21 bar at 873 K, and ca. 10−15 bar at 1073 K. Before reaching this PO2, the gradual decomposition of the Fe rich compounds was observed by atmosphere controlled high temperature XRD. From the results of the measurement procedure dependence of the coulometric titration data, the actual decomposition PO2 of the Fe rich compounds can be considered almost comparable to those of the Co rich compounds.