The enzymology of an activated sludge system fora petroleum wastewater purification process was investigated.Leucine-aminopeptidase (L-AMP), β-glucosidase (β-GLC),and lipase (LIP) were selected for the study. It was found thatmore than 81.7% of enzymatic activity was associated withmicrobial cells in the activated sludge floc. The metabolicconcentration showed that L-AMP activity increased in theactivated sludge, whereas activities of β-GLC and LIP decreased,due to the inhibitory effect of the phenol which varied from100 mg/l to 50 mg/l.