An experimental study of the oxygen non-stoichiometry and redox thermodynamics of LaMn1 − xCoxO3 ± δ (x = 0.25 and 0.75) is presented. The system shows partial solid solubility of LaCoO3 in LaMnO3 and vice versa. While both samples consist of two phases at room temperature, quenching experiments show that the x = 0.25 sample is single phase at high temperatures for the whole range of study, 0.6 < − log(pO2/atm) < 14. The x = 0.75 sample consist of two phases even at high temperatures. This does not necessarily represent a problem for applications, and both samples are studied in details. The oxygen non-stoichiometry versus oxygen partial pressure are measured at three temperatures of 1223, 1273, and 1323 K by coulometric titration and thermogravimetry, for x = 0.25 and 0.75, respectively. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and are − 680 ± 40 and for x = 0.25 and 0.75, respectively. These data and analyses of literature data for LaMnO3 and LaCoO3 show that the enthalpy of oxidation varies nearly linearly with x. Solid solution thermodynamic models are used to analyze the redox mechanism.