Because the polycyclic compounds are the most toxic from the ecological point of view, the study of their changes during oil degradation processes in the environment is of interest. During the oil degradation in soil the PAH content decreases both in oil and tar fractions. The decrease in 2-6 PAH rings amounts to 33.5% for oil and 49.8% for tar fraction. The appropriate numbers for NPD are 34.5% and 49.4%, and for 16 EPA 48.8% and 43.2%. In the oil degradation process the asphaltene content increases from 0.1% to 6.5%, i.e. 65 times in its composition. Apparently, the reduction in PAH in the composition of oil and tar is associated with the fact that the heavy part of oil hydrocarbons polycondenses and separated into asphaltenes. Moreover, the greater the molecular weight and number of aromatic weight, the more quickly the polycondensation process occurs than in light PAH. In Figs. 1 and 2, it is shown the dependence of the ratio of the individual PAH concentrations in the composition of oil and tar fraction of the original and degraded oil on the number of benzene rings and molecular weight. Fig. 1. Fig. 4.It is seen that with increase of the number of rings and molecular weight the ratio of Сt/Coil increases, which indicates the behavior of polycondensation processes. In this case, these processes occur more quickly in oil fraction and the content of PAH decreases in lighter oil fractions. On the other hand, PAH are more soluble in water at smaller amounts of aromatic rings and accordingly, they are subjected significantly to biodegradation by microorganisms. Salt and humus content in soil also affects the dissolution process positively. Thus, a sharp decay in the curves of Fig. 2, is apparently related to a higher rate of degradation of relatively light (k < 4) PAH than heavy ones. Moreover, it should be also considered the role of direct and indirect photochemical decay of PAH under solar radiation.