The response of activated sludge to the presence of 2,4-dichlorophenol (dCP) was investigated in this study. At 1-20 mg/l, dCP slightly reduced the specific oxygen uptake rate of the activated sludge. The sludge yield slightly decreased with an increase in dCP concentration, and its hydrophobicity also changed. The presence of dCP at 5 mg/l or lower did not change the diameter distribution of the sludge flocs. However, the diameter of most floc increased at dCP higher than 5 mg/l. Energy uncoupling and inhibition analysis demonstrate that the reduction in sludge yield at lower dCP levels was mainly attributed to the inhibitory effect of dCP on microbial growth, while that at higher levels of dCP, the yield reduction was mainly caused by the inhibitory effect of dCP on the substrate degradation. (c) 2006 Elsevier Ltd. All rights reserved.