The heat in heavy oil pyrolysis process is a quantity of interest in the design of thermal process equipments and the calculation of energy balance in refineries. Thus, enthalpy of reaction and enthalpy of vaporization in the pyrolysis of four heavy oils: DQVR (Daqing vacuum residue), KRVR (Karamay vacuum residue), LHVR (Liaohe vacuum residue) and VNVR (Venezuela vacuum residue), have been quantitatively studied by a combination of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), plus microreactor experiments. As sample became heavier, the temperature range of pyrolysis process increased from 172 to 200 degrees C and enthalpies of reaction decreased, which were determined to be -1070, -570, -489 and -393 J/g for DQVR, KRVR, LHVR and VNVR, respectively. The enthalpies of reaction at various conditions showed a linear trend with respect to the final coke yield in the thermal analysis. Enthalpies of vaporization in the thermal analysis process were calculated to be 188, 181, 161 and 128 J/g, for DQVR, KRVR, LHVR and VNVR, respectively. The contribution of vaporization to the overall pyrolysis process varied between 0.12 and 0.19. A method to quantify the enthalpy of cracking and that of condensation was tentatively developed by using extrapolation. (c) 2012 Elsevier B.V. All rights reserved.