Studying oil behavior in natural geosystems, aimed at identifying its transformation laws is a very urgent and little-studied matter, especially in Azerbaijan, where oil production is an important part of its economy. Solving this problem could serve as the theoretical basis of organization of oil-pollution monitoring and the efficient recovery of oil-polluted biocenoses. Under the laboratory experiment conditions features of transformation of the chemical composition of oil pollution in the process of microbial degradation in soil were studied. The research object was oil pollution with different formation time from the areas of operation of oil wells located in the territories of Absheron Peninsula. There were estimated the level of organic pollution (3-20%), physicochemical properties and microbial diversity of the investigated soil samples. The analysis of the destructive activity of selected individual strain-destructors and their associations against oil and oil products degradation was conducted. Changes in the fractional composition of oils extracted from polluted soils after their biodegradation with various microorganisms in liquid nutrient media and in soils, reflecting the quantitative and qualitative changes of the oil composition were evaluated. By use of the method of gas chromatography-mass spectroscopy in oil and oil products of the studied soils there were found fragments of relict hydrocarbons. Normal and isoprenoid alkanes, mono-, bi- and polynaphthenes, steranes, hopans as well as some aromatic hydrocarbon markers were studied. The revealed high resistance of sterane (m/z 217) and hopan (m/z 191) in the polluted soil allows us to recommend their use as biomarkers for evaluation of the source of oil pollution, establishing the source of oil, polluting the soil, as well as control over the process of bioremediation of oil-polluted soils.