Thermolysis of crude oil at a temperature of 350A degrees C for 1 h with and without aluminosilicates has been conducted in the isothermal regime mode. The composition of liquid and gaseous thermolysis products has been analyzed. The amount of resins and asphaltenes in the liquid products of thermal or thermocatalytic treatment decreases and that of oils increases as compared with the crude oil. The individual composition of saturated hydrocarbons (alkanes, steranes, terpanes) and alkylaromatic compounds (benzenes, naphthalenes, and phenanthrenes homologoues) has been studied. It has been shown that thermolysis increases the amount of thermodynamically more stable m- and p-isomers of alkyltoluenes, and the beta-isomers of alkyl-naphthalenes and alkylphenanthrenes. Geochemical parameters for the composition of alkyl substituted benzenes, naphthalenes, and phenanthrenes have been calculated.