The chemical composition of high-viscosity Mongolian oil and the influence of surfactant-based, composite oil displacement agents on the distribution of hydrocarbons (HCs) during thermostating of the crude oil-agent (oil-water) system under laboratory conditions have been studied. It has been shown that the presence of the agent in the system leads to redistribution of hydrocarbons, a change in the HC composition of the oil phase, and an increase in the concentration of petroleum components in the aqueous phase. The increase is especially noticeable when a surfactant is used: the proportion of light C-10-C-15 alkanes increases in the oil and C-16-C-25 alkanes and cyclohexanes dominate in the aqueous phase.