Perovskite type LaMnO3 and related materials are important compounds with many useful and unique physical and chemical properties. There is a lack of experimental thermochemical data on the energetics of LaMnO3 nanoparticles. In this work, a series of LaMnO3+δ samples were synthesized via the citrate method and calcined at 700°C–1050°C. All samples displayed rhombohedral structure (X-ray diffraction) with similar oxygen stoichiometry 3+δ = 3.16–3.18 (iodometric titration coupled with gravimetric analysis). The BET surface area varied from null for bulk sample to 6.88 ± 0.08 × 103 m2/mol for the sample calcined at 700°C. The water content varied linearly with the surface area with the highest value being 2.34 wt%. The chemisorbed water adsorption enthalpy was −63.0 ± 4.1 kJ/mol with the chemisorbed water coverage of 8 H2O/nm2. High-temperature oxide melt drop solution calorimetry, performed in sodium molybdate at 702°C, yielded enthalpy of formation from La2O3, Mn2O3, and O2 of bulk LaMnO3.16 of −77.85 ± 1.94 kJ/mol. After correction of drop solution enthalpies of nanometric samples for water content, the calorimetric data were used to calculate the surface energy of LaMnO3+δ. The energy of the anhydrous surface was 2.27 ± 0.29 J/m2, and that of the hydrous surface was 2.02 ± 0.27 J/m2. These values are higher than the surface energies of LaMnO3.00 predicted elsewhere by theoretical methods, probably due to the different oxygen content and possibly more complex surface structure and exposed surface planes. The measured surface energy of LaMnO3+δ lies between the values reported recently for BaTiO3 and PbTiO3 and close to the reported values for MnO2. This suggests that LaMnO3+δ surface is predominantly MnO2-terminated, in line with the trends predicted by theoretical calculations.