The oxygen non-stoichiometry and redox thermodynamic properties of the LaFe1 − xCoxO3 − δ system (x = 0.25 and 0.75) are studied. At low temperatures, the LaCoO3 and LaFeO3 systems show partial solid solubility. At 1,273 K (in air), both compounds are single phases and are orthorhombic and rhombohedral for x = 0.25 and 0.75, respectively. Thermogravimetry has been used to measure the oxygen non-stoichiometry versus oxygen partial pressure at three temperatures, 1,223, 1,273, and 1,323 K. Redox thermodynamic quantities are extracted directly from the oxygen non-stoichiometry curves. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and for x = 0.25 and 0.75, they are −640 ± 60 and −440 ± 60 kJ (mol O2)−1, respectively. Ideal solid solution thermodynamic models are used to analyze the redox mechanisms.