Barret et al. stated, that the hydrocarbons anaerobic digestion was not strongly related to the overall metabolism of the biomass. In this research the total dehydrogenase activity assessment was applied for the evaluation of microbial enzymatic activity. The results obtained in the presence of the diesel fuel were similar to the results for the control bioreactor, both in endoge-nous and substrate-supplied samples In case of engine oil, the visible decrease in the microbial dehydogenase activity occurred.The similar tendencies in enzymatic activity in the control and experimental reactors were observed for both petroleum products.