The energy of combustion of a pure sample diphenyl oxide in crystal and liquid states at T = 298.15 K was determined by isoperibolic calorimeter combustion. The enthalpy of fusion, the triple-point temperature and the purity of the sample were estimated by studying the melting process by calorimetric method. The enthalpies of formation in the crystalline, liquid and gaseous states were calculated. All results are compared with the experimental values reported in the literature. The value of gas-phase enthalpy of formation was calculated by G4 method in combination with isodesmic reactions. The results of this calculation are in good agreement with the obtained experimental value. (c) 2018 Published by Elsevier Ltd.