Hydrogen peroxide was detected during the ozonation of para-chlorophenol (4-CP) in aqueous solutions. Its formation and influence on the reaction mechanism were examined. Semi-batch experiments showed that hydrogen peroxide was formed during the direct reaction between molecular ozone and 4-CP. The detected hydrogen peroxide could reach 12.3% of the initial mole concentration of 4-CR Hydrogen peroxide reacts with ozone in neutral pH and leads to the appearance of the powerful oxidant, OH center dot radicals. An O-3/OH center dot probe compound, succinate was then developed. and also included in the 4-CP solution matrix. The disappearance of succinate with time shows the amount of OH center dot radicals. With enough tert-butanol inhibiting the OH center dot radical oxidation, results showed that 4-CP was transformed to para-quinone by ozone, and subsequently destroyed to formic acid and oxalic acid. Without scavengers of inactivation the OH center dot radicals, 4-CP was destroyed to low molecular weight acid. (c) 2006 Elsevier B.V. All rights reserved.