Abstract <p>The main types of soils in the northeastern part of the Sakhalin island have been studied under oil pollution conditions (1 and 10 years) vs. their background counterparts. Patterns of hydrocarbon transformation have been revealed in oil-contaminated soils; the possibility of launching self-purification processes is shown based on the results of assessment of the composition of petroleum hydrocarbons (TPHs). Analysis of <i>n-</i>alkanes as markers of TPH biodegradation has shown a change in their content and composition, depending on the degree and age of soil contamination with TPHs, as well as the sorption capacity of soils. Mechanisms of soil self-purification are triggered during the first year, which leads to a change in the composition of <i>n</i>-alkanes: light fractions volatilized and C16–C23 alkanes undergo biodegradation. Under conditions of 1-year oil contamination of soils, the composition of petroleum hydrocarbons undergoes significant changes, which are expressed in an increased proportion of microbial <i>n</i>-alkanes (ΣC20–25). An increase in the content of high molecular homologs ΣC26–C34 has been found compared to the initial state of oil-contaminated soils and the oil composition. The calculations of <i>CPI</i> (coefficient of oddness), <i>K</i>_i (coefficient of biodegradation), and <i>K</i>_w (“degree of waxiness”) demonstrated the transformation of the composition of <i>n-</i>alkanes and general transformation of petroleum hydrocarbons in time. The use of the sum of odd <i>n-</i>alkanes C27, C29, and C31 as a diagnostic indicator is a solid base for selecting background soils in the zone of hydrocarbon deposits.</p>