Changes in the properties and chemical structure of high-molecular-mass heteroatomic compounds (resins, asphaltenes) as a result of the thermal treatment of heavy oils from the Zuunbayan (Mongolia) and Usinsk (Russia) oil fields have been shown. The structural-group parameters of resins and asphaltenes isolated from crude oils and their thermolysis products have been determined on the basis of elemental analysis, molecular mass, and (1)H NMR data. It has been shown that the yield of gaseous, liquid, and solid products varies depending on the thermolysis time and temperature. A model for the thermal conversion of the high-molecular-mass components of petroleum is proposed and the rate constants (k) and activation energies (E) of the petroleum thermolysis reactions have been calculated. The competing routes of the conversion of petroleum components at different temperatures have been established.