Determination of the surface energy of materials at nanometric scale has remained a thorny problem mainly because this energy is shaded by the species absorbed on the surface of material, particularly water. By knowing the energy contribution of water to the surface one can assess the anhydrous surface energy. Using the custom combination of gas dosing system and microcalorimeter allows precise simultaneous measurement of amount of water adsorbed and produced heat effect. Using this method water adsorption energetics of fluorite-structured CeO2and ThO2was measured for the first time. CeO2and ThO2were studied at both, bulk and nano scale with nanoparticles prepared via surfactant-based method. We found that 8.8 ± 0.4 H2O molecules/nm2for CeO2and 8.2 ± 0.3 H2O molecules/nm2for ThO2chemically react with the oxide surface. The enthalpies of these reactions are more exothermic than the condensation of water. The total integral adsorption enthalpy is -59.8 ± 0.7 kJ/mol and -65.0 ± 1.2 kJ/mol, for CeO2and ThO2respectively (standard state is water vapor). Surface energies for CeO2and ThO2were estimated using high temperature oxide melt solution calorimetry from changes in enthalpy with surface area. Enthalpies of anhydrous surfaces are 1.16 ± 0.02 J/m2for CeO2and 1.21 ± 0.03 for ThO2. The comparison of the adsorption and the surface energetics with the results of theoretical calculations will be discussed.