The effect of soil contamination with diesel oil and petrol on the nitrification process was investigated in a laboratory experiment. Samples of typical brown soil developed from loamy sand, of pH of 6.6 in 1M KCl, Hh –11.38 mmol+ kg–1 soil, S – 77.67 mmol+ kg–1 soiland Corg – 8.50 g kg–1 were analyzed. The experiment was performed in three replications, and for each test 100 g air-dry soil sample was placed in 150 cm3 beakers. Soil samples were contaminated with diesel oil and petrol with the addition of rapeseed oil and ethanol. The source of nitrogen was ammonium sulfate in the amount of 0 and 250 mg N per kg–1 soil. The content of N-NO3 and N-NH4 was determined on experimental days 14, 28 and 42. Soil moisture was kept constant at 50% capillary water capacity throughoutthe experiment. Fertilizer nitrogen was subject to strong immobilization in soil contaminated with dieseloil and petrol. Both pollutants strongly inhibited the nitrification process. Diesel oil hada much stronger inhibitory effect on nitrification than petrol. Rapeseed oil also proved tobe a powerful inhibiting factor. On experimental day 42, diesel oil reduced ammonium cationoxidation by 99%, and petrol – by 88%.