The degradation of 4-chloro-2-methylphenol (PCOC) in aqueous solution by UV irradiation in the presence of TiO2 has been studied. It was found that only 51.4% of PCOC was degraded by this process. The improvement in degradation levels were achieved by using various oxidants as electron scavenger. Periodate ion was found to be much more efficient oxidant than the others in improving the degradation of PCOC via formation of reactive species. The effect of the oxidants used in the UV/TiO2 systems on the degradation rate of PCOC were found to be in the order of IO4- > BrO3- > H2O2 > O-2 > ClO3-. Addition of metal ions to UV/TiO2/H2O2 system increased the photodegradation rate of the organic substrate. The effect of Mn2+, Cr3+, Fe2+ and Fe3+ metal ions on degradation of PCOC in the TiO2/UV/H2O2/Mn+ system was found to be in the order of Fe3+ > Fe2+ > Mn2+ > Cr3+. Langmuir-Hinshelwood rate expression was employed for the degradation of PCOC by the UV/TiO2/O-2 system and adsorption equilibrium constant and the rate constant were determined to be 74.16 mM(-1) and 0.0081 mM min(-1), respectively. (C) 2004 Elsevier B.V. All rights reserved.