The standard Gibbs energy of formation of LaMnO₃ has been measured in the temperature range from 900 to 1400 K using a solid-state cell incorporating a buffer electrode. The oxygen potential corresponding to the three-phase mixture MnO + La₂O₃ + LaMnO₃ has been measured using pure oxygen gas at 0.1 MPa as the reference electrode, and yttria-stabilized zirconia as the solid electrolyte. The buffer electrode prevented polarization of the three-phase electrode and ensured accurate data. The measured oxygen chemical potential corresponding to the decomposition of LaMnO_3−δ to MnO, La₂O₃ and O₂ gas is in fair agreement (±6 kJ mol⁻¹) with most data available in the literature and can be represented by the equation,Δμ_O2/J mol⁻¹ (±520) = −583160 + 170.55 (T/K)Combining this with available information on oxygen potential variation with the non-stoichiometric parameter δ, the standard Gibbs free energy of formation of LaMnO₃ from MnO, La₂O₃ and O₂ was evaluated. The correction for non-stoichiometry was ∼1.6(±0.7) kJ mol⁻¹. The heat capacity of LaMnO₃ has been measured by a differential scanning calorimeter from 400 to 1060 K. Information on low-temperature heat capacity available in the literature was joined smoothly with the high-temperature data to evaluate the standard entropy of LaMnO₃ at 298.15 K as 116.68 J mol⁻¹ K⁻¹. The “third-law” analysis of the Gibbs energy values obtained in this study yields a value of −155.93 (±0.8) kJ mol⁻¹ for the standard enthalpy of formation of LaMnO₃ from MnO, La₂O₃ and O₂ at 298.15 K. This value provides a critical test of calorimetric data on enthalpy of formation of LaMnO₃. A consistent set of thermodynamic data for LaMnO₃ is presented from 298.15 to 1400 K based on the results obtained in this study and other supporting information in the literature.