Changes in activity of amylase, cellulase, protease and dehydrogenase during biodegradation of petroleum contaminants were investigated in laboratory studies. Refinery soil polluted for a long time was modified by commercial biological preparation and natural biological preparation (prepared from this soil). These modifications did not influence cellulase and protease activity. Activity of amylase and dehydrogenase changed in these samples. In the sixth week the highest activity of dehydrogenase was noticed, especially in soil with commercial biological preparation. Modifications had a great influence on contaminants degradation. The highest elimination of TPH (20,4%) and heavy fractions (10,5%) took place in soil modified by commercial biological preparation. The highest PAH elimination was in soil with natural bacterial preparation