Samples of soil contaminated with mazut (residual oil) were a source for isolation of mazut-degrading microorganisms. Most of the strains isolated were characterized taxonomically as members of the Pseudomonas genus. Qualitative and quantitative changes (CHN-composition and weight) in mazut in the course of its biotransformation in liquid media and soil were analysed. More detailed information was obtained by the mathematical analysis of Fourier transform infrared (FTIR) spectra for the determination of mazut fractions prior to and after bacterial degradation in liquid media. Some strains could degrade about 20% of mazut in liquid media after a 10 day incubation and about 30% in soil after a 20 day incubation. Data on the activity of degraders with respect to mazut composition allow a set of bacterial associations degrading oil and mazut to be established.