The effects of di-n-butyl-phthalate (DBP) on both acclimated and unacclimated activated sludge were investigated in batch reactors. The IC50 values on the basis of maximum specific growth rate (mu(m)), the chemical oxygen demand (COD) removal efficiency and the oxygen uptake rate (OUR) of unacclimated activated sludge was determined and found to be 78.8, 52.2 and 50.1 mg/l, respectively. The COD removal efficiency was affected even at low DBP concentrations. Activated sludge acclimated to 300 mg/l DBP could tolerate about 500 mg/l DBP based on the COD removal efficiency. Although unacclimated activated sludge could not degrade DBP, the acclimated activated sludge could degrade up to 100 mg/l DBP completely. The results showed that the acclimated activated sludge could use DBP as sole organic carbon source. The degradation rate of DBP by acclimated activated sludge could be described by the first-order kinetics equation. (C) 2003 Elsevier Ltd. All rights reserved.