The thermodynamic properties of the perovskite compounds La0.8Sr0.2MnO3, La0.9Na0.1MnO3 and LaMnO3 were studied by use of the solid electrolyte galvanic cell method at 1000-degrees-C, 1050-degrees-C and 1100-degrees-C. Two samples of each compound were investigated as well as decomposed samples of La0.8Sr0.2MnO3 and La0.9Na0.1MnO3. The cell assembly was constructed by means of eight small stabilized zirconia tubes and a common Ni/NiO reference electrode. The equilibrium partial pressures of the samples were calculated from the measured EMF values. The results reveal that the equilibrium PO2 Of LaMnO3 appears to be one or two orders of magnitude lower than that of La0.8Sr0.2MnO3 and La0.9Na0.1MnO3, respectively, which means that LaMnO3 is more stable than the doped perovskites. The variation in the standard free energy with temperature for the perovskite decomposition reaction was calculated from the pO2 equilibrium values, i.e. DELTAG-degrees d(LA0.8Sr0.2MnO3) = 140.86 - 0.05199T kJ/mole and DELTAG-degrees d(La0.9Na0.1MnO3) = 106.06 - 0.02572T kJ/mole. On the basis of the above equations, the reaction enthalpy and entropy changes were calculated.