The effect of EOR system in the laboratory on the composition of high-viscosity and heavy oils from the oil fields of  Russia, Germany and Mongolia was investigated. It is shown that thermal treatment and influence to the components of the EOR system causes a significant change in the composition and properties of the investigated oils. The content of low molecular weight n-alkanes in the treated oils was increased 2.5 times. Viscosity of oils was reduced almost three times, the amount of oxygen-containing compounds and total acid number of oils has also declined.