The enzymology of an activated sludge system for a petroleum wastewater purification process was investigated. Leucine-aminopeptidase (L-AMP), ${eta}$-glucosidase (${eta}-GLC$), and lipase (LIP) were selected for the study. It was found that more than 81.7% of enzymatic activity was associated with microbial cells in the activated sludge floc. The metabolic response of a mixed microbial population to increased phenol concentration showed that L-AMP activity increased in the activated sludge, whereas activities of ${eta}-GLC$ and LIP decreased, due to the inhibitory effect of the phenol which varied from 100 mg/l to 500 mg/l.