The present paper is devoted to the results of long-term field observations of oil pollution transformation in small watercourses of the Lower Don River basin (Russia), on the Atlantic coast of France and in the Strait of Kerch polluted by spilled oil. The investigations included visual assessment and sampling of oil slicks, water and sediment cores. The analysis of samples were carried out with the use of thin layer and column chromatography, optical and gravimetric methods making possible to determine separately the sum of saturated, mono- and di-aromatic hydrocarbons (HC), the sum of polycyclic aromatic hydrocarbons (PAH) and the sum of asphaltic components (AC) as well as the presence of hydrocarbons of contemporary biological origin. In the course of time it’s revealed an exponential decrease in the oil pollution level and in the ratio between hydrocarbons and asphaltic components (HC/AC). We suggest that the ratio HC/AC can be considered as a simplified indicator of the oil pollution transformation level as it takes into account only qualitative pollutant changes and is not considerably influenced by dilution, migration and other mechanical factors. In the studied cases the values of half-period of oil pollution transformation varied from 350 to 6930 days. The highest activity of natural cleansing process was observed on the exposed rocky shores, sandy and gravel beaches and riverine sectors with coarse sediments, rapid water flow and high oxygen saturation. The statistical analysis of the results of long-term observations enabled the authors to quantify the influence of principal geographical factors on the rates of natural cleansing process. This information is used to develop an innovative methodology of estimation of water ecosystems’ natural cleansing ability.