Abstract <p>The patterns of transformation of <i>n-</i>alkanes as biomarkers were studied to assess dynamic of transformation of petroleum products (PPs) in oil-contaminated soils of the chestnut series (Stavropol krai). The study revealed patterns of hydrocarbons transformation in oil-contaminated soils and the possibility of launching self-purification processes based on the results of an assessment of the PP composition. Medium-molecular <i>n-</i>alkanes dominated in the oil composition. The absence of high-molecular-weight <i>n-</i>alkanes (heavy fractions) and the presence of liquid ones (part of the kerosene fraction) were the main features of the hydrocarbon state in background uncontaminated soils. This indicated the existence of the regional background on the basis of the content of PPs in soils. Homologous series of <i>n-</i>alkanes of freshly oil-contaminated soil samples included compounds with a number of carbon atoms from C₁₁ to C₄₀, which contained markers of organic matter of hydrobiont genesis (C₁₅, C₁₇, and C₁₉), markers of organic matter of bacterial genesis (C₂₀–C₂₅), and markers of terrigenous humic organic matter (C₂₇, C₂₉, and C₃₁). Normal alkanes with the long carbon chain C₂₀–C₂₅ and C₂₆–C₄₀ were represented in almost equal proportions with a slight predominance of the latter. It was shown that, 4 months after the moment of crude oil contamination, the results of soil self-purification were recorded, leading to a change in the composition of <i>n-</i>alkanes. The biodegradation of <i>n-</i>alkanes in soils was confirmed by an increase in the carbon of microbial biomass in oil-contaminated soils relative to the background analogues. The nature of the distribution of even and odd homologues of <i>n</i>‑alkanes and a value of the sum of odd <i>n-</i>alkanes (ΣC₂₇, C₂₉, C₃₁) are advisable to evaluate along with the total content of PPs when choosing background analogues of soils.</p>