This paper presents transformations of saturated hydrocarbons of petroleum type pollutants during ex situ bioremediation of soil on the pilot heap (halde), during a period of 6 months, within the grounds of Petroleum Refinery Pančevo (Serbia). Samples for analysis were taken in time intervals of 2 weeks (P 1 -P 12 samples). Organic substance was extracted by Soxhlet's method and quantified. Isoprenoid aliphatics, in particular pristane and phytane, and polycyclic aliphatics of sterane and triterpane types in saturated hydrocarbon fractions were analysed by GC-MS (SIM method). Significant amounts of n-alkanes have not been detected. The MSchromatogram revealed only marginal amounts of pristane and phytane in sample P 1 . Pristane and phytane occurred in sample P 8 , and in even higher quantities in the final sample P 12. The proceeding bioremediation process was accompanied by the decrease of the relative amounts of pentacyclic terpanes of hopane type, compared to tri-and tetracyclic terpanes. In the initial sample P 1 the distribution of steranes and hopanes follows a pattern, which is characteristic for crude oils. However, their identification by SIM method was not possible in samples P 8 and P 12 because of the reduced concentration. The observed changes in the alkane fractions' compositions may be considered as atypical, referring to the fact that during oil biodegradation under natural conditions, decomposition of isoprenoids occurs much easier and faster than decomposition of polycyclic alkanes of tri-, tetra-and pentacyclic terpane, sterane and diasterane types, after the decomposition of n-alkanes has been almost completed.