The impacts of 2- chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) on the kinetics of biogenic substrate utilization by biomass from sequencing batch reactors (SBRs) were investigated. Experiments were carried out with a mixed unacclimated biomass from two SBRs with sludae ages of 5 and 8 days, using a respirometric method. The EC50 values of 2-CP and 2,4-DCP for the biomass from SBRs with sludge ages of 5 days and 8 days were found to be 141 and 163 mgl(-1), and 40 and 91 mgl(-1), respectively. Test compounds induced a stable inhibition effect with time on target microorganisms. 2-CP exhibited uncompetitive inhibition whereas 2.4-DCP exhibited noncompetitive inhibition according to the maximum oxygen uptake rate (OURmax) and K-s trends. 2-CP and 2,4-DCP both exerted higher inhibition effect on biomass with 5 days of sludge age. (c) 2007 Elsevier Ltd. All rights reserved.