A solid-state electrochemical technique was employed to study the thermodynamic properties of some nonstoichiometric undoped and 30% Sr-doped LaMnO3 perovskites over the temperature range 1073-1373 K. The equilibrium pressures of oxygen and the relative partial free energies, enthalpies and entropies of oxygen in LaMnO3+/-x and La0.7Sr0.3MnO3+/-x were determined as a function of temperature and oxygen nonstoichiometry.