By performing DFT calculations, the reaction of 4-chlorophenol with (OH)-O-center dot is reexamined to reconcile an experimental finding with a recent theoretical result. The calculated results show that abstracting hydrogen atom in the hydroxyl of 4-chlorophenol by the (OH)-O-center dot is the most plausible process for forming 4-chlorocaechol intermediate, while adding (OH)-O-center dot to the aromatic ring is the dominant pathway for forming the hydroquinone. The 4-chlorophenoxyl radical plays a crucial role during the hydroxyl-initiated 4-CP degradation due to its energetic stability and the low barrier involved in the reaction, which supports the experimental finding but differs from the recent theoretical result. (C) 2009 Elsevier B. V. All rights reserved.