Abstract The thermal decomposition of solid samarium(III) oxide telluride (SmO)2Te was studied over the temperature range 1788–1972 K by the Knudsen effusion weight-loss technique. The compound decomposes to yield B-Sm2O3(s) and a gas phase containing samarium and tellurium in a molar ratio of 2:3. Using enthalpy and entropy data from the literature for gaseous Sm, Te, SmTe and Te2, it could be established from thermodynamic calculations that, within the given temperature range, samarium and tellurium are the principal vapour species in equilibrium with solid (SmO)2Te and B-Sm2O3 (monoclinic modification), while SmTe and Te2 are of minor and very minor importance respectively. Using estimated values for the standard entropy and heat capacity of (SmO)2Te, the following second-law (index II) and third-law (index III) enthalpies and entropies for the reaction (SmO) 2 Te(s) → 2 3 Sm 2 O 3 (s) + 2 3 Sm(g) + Te(g) were found: ΔH298 °(II) = 679.0 ± 10.0 kJ mol−1; ΔH298°(III) = 686.7 ± 19.7 kJ mol−1; ΔS298°(II) = 208.9 ± 6.0 J mol−1 K−1; ΔS298°(III) = 213.1 J mol−1 K−1. A value of −1545 ± 15 kJ mol−1 was derived for the standard enthalpy of formation of (SmO)2Te. Possible trends of the enthalpies of formation of all compounds (LnO)2Te (Ln ≡ La-Lu) are discussed.