Abstract The degradation of 4-chloro-2-methylphenol (PCOC) in aqueous solution by UV irradiation in the presence of TiO 2 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/TiO 2 systems on the degradation rate of PCOC were found to be in the order of IO 4 − > BrO 3 − > H 2O 2 > O 2> ClO 3 −. Addition of metal ions to UV/TiO 2/H 2O 2 system increased the photodegradation rate of the organic substrate. The effect of Mn 2+, Cr 3+, Fe 2+ and Fe 3+ metal ions on degradation of PCOC in the TiO 2/UV/H 2O 2/M n+ system was found to be in the order of Fe 3+ > Fe 2+ > Mn 2+ > Cr 3+. Langmuir–Hinshelwood rate expression was employed for the degradation of PCOC by the UV/TiO 2/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.