This study investigated, for the first time, thermal transformations of heavy oil asphaltenes using a stepwise thermal decomposition method under conditions that enable a researcher to properly consider variations in the bond energies of asphaltene molecules and to minimize the occurrence of secondary reactions with newly-formed products. Based on the thermolysis material balance, it was found that at temperatures up to 290 degrees C, the asphaltene conversion rate exceeds 90%, and asphaltene transformations involve the formation of considerable amounts of gaseous compounds, liquid hydrocarbons, and resins, the total content of which reaches 50 wt %. The structural variations in asphaltenes during thermolysis were evaluated by H-1 NMR spectroscopy, elemental analysis, and cryoscopic measurement of average molecular weight in naphthalene. It was demonstrated, using various physicochemical analytical methods, that the stepwise thermolysis of asphaltenes is accompanied by a 1.5-fold increase in the average molecular weight of their molecules due to recombination reactions of newly-formed macroradicals. After thermolysis at 230 degrees C, all the tested asphaltenes display an almost identical distribution of carbon atoms among the aromatic, naphthenic, and paraffinic fragments of their molecules regardless of the composition and structure of the initial asphaltenes. The asphaltene reactivity up to 230 degrees C is determined by the number of sulfur- and oxygen-containing fragments labile under the imposed conditions. At higher temperatures, the key feature crucial for asphaltene reactivity is the carbon skeleton structure of asphaltene molecules.