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The oxygen content was measured in cubic perovskite-type La 1/3 Sr 2/3 Fe 1- x Mn x O 3- δ ( x = 0.1, 0.17, 0.25, and 1/3) in the range of oxygen partial pressure from 10 -22 to 0.5 atm at 750-950 °C with a step of 50 °C by coulometric titration. Gradual removal of oxygen from the oxides during the measurements was carried out until the stability limit was achieved and the reductive decomposition began. An increase in manganese content was shown to lead to a decrease in the stability of La 1/3 Sr 2/3 Fe 1- x Mn x O 3- δ under reducing conditions. The obtained data on oxygen content were used for defect chemistry modeling in the oxides. The enthalpy of the Fe 3+ to Fe 4+ and Mn 3+ to Mn 4+ oxidation reactions (Δ H ox 0 ) was determined to be -103.2 ± 0.3 and -250 ± 2 kJ mol -1 , respectively, for the x = 0.1 composition, and increased slightly with increasing manganese content. The large difference in Δ H ox 0 determines a strong distinction between the behavior of iron and manganese in perovskite-type oxides. An increase in manganese content in La 1/3 Sr 2/3 Fe 1- x Mn x O 3 -δ was found to lead to a decrease in the concentration of Fe 4+ ions, but did not affect the concentration of Fe 2+ ions. The impact of La/Sr ratio was evaluated by comparison of the obtained data with that for La 0.5 Sr 0.5 Fe 1- x Mn x O 3 -δ , and found to be different for iron and manganese. An increase in lanthanum fraction causes a decrease in the concentration of Fe 2+ ions and an increase in the concentration of Mn 2+ under reducing conditions.