In an attempt to enhance the oxidation of pentachlorophenol (PCP) in the Fe(III)-PcTS/KHSO5 System, the presence of added humic substances (HSs) was studied, investigating the chemical properties of HSs related to the enhancement in PCP oxidation by correlations with the degree of enhancement in PCP oxidation (%delta(PCP)(60)). The %delta(PCP)(60) value increased with a decrease in the content of oxygen-containing functional groups, such as carboxylic acids. This indicated that HSs with a lower content of oxygen-containing functional groups would be useful for enhancing the oxidation of PCP. A negative correlation between %delta(PCP)(60) and the kinetic constants of Fe(III)-PcTS self-oxidation indicated that the enhancement by added HSs could be attributed to the suppression of Fe(III)-PcTS deactivation by self-oxidation. Such a stabilization of Fe(III)-PcTS could be attributed to hydrophobic interactions between the catalyst and HSs. (c) 2005 Elsevier Ltd. All rights reserved.