The kinetics of thermal degradation of high-molecular-mass fractions of crude oil mixed with rapeseed oil has been studied by dynamic thermogravimetry. The structural parameters of the average molecule of vacuum residues of crude oils processed at Naftan and the Mozyr refinery and hydrocarbon oils, resins, and asphaltenes isolated from the residues have been first determined by NMR and IR spectroscopy. It has been shown that the addition of rapeseed oil can increase the rate of thermal degradation of heavy petroleum residues, with the character of the effect of the vegetable oil on the kinetics of the process being determined by structural features of macromolecular petroleum compounds, namely, the relative amount of aromatic units and the length of alkyl chains.