In this work, the effect of butyl bromide on the thermal transformations of heavy oil from the Karmalskoye field (Republic of Tatarstan) in the presence of an initiating additive, n-butyl bromide, was studied. It was shown that the addition of butyl bromide increased the yield of gasoline and diesel fractions due to the destruction of high-molecular-weight components. It was established that almost all bromine from butyl bromide entered the compaction products upon the cracking of heavy oil, and butyl radicals entered the composition of gaseous products. It was noted that the direction of thermal transformations of hydrocarbons changed in the presence of butyl bromide. As compared to the initial oil, the amount of low-molecular-weight alkanes and isoprenoids increased significantly, the concentration of cyclohexanes and cyclopentanes decreased, and tri-, tetra- and pentacyclic saturated hydrocarbons were completely destroyed.