Photocatalytic degradation of 4-chlorophenol (4-CP) was studied using TiO2, Pt/TiO2, Ag/TiO2 prepared by the sol-gel methods and Degussa P25 as photocatalysts. The influence of dissolved oxygen on the reaction rate and amounts of intermediate products were determined. In the experiments, a photocatalyst was suspended in the 4-CP solution, which was then irradiated with an 11 W low pressure mercury lamp emitting UV light in 200-280 mn window. The results show that, with TiO2 (sol-gel), the decrease of 4-CP concentration was much faster than that with Degussa P25. In contrast, the reduction rate of total organic carbon (TOC) with Degussa P25 was much higher than that with TiO2 (sol-gel). The addition of a small amount of either Pt or Ag into TiO2 (sol-gel) improved the catalyst activity significantly, the highest activity being obtained with 1.0% Pt/TiO2 and 0.5% Ag/TiO2. 0.5% Ag/TiO2 showed the highest activity in terms of both 4-CP and TOC removals. Hydroquinone (HQ) and hydroxyhydroquinone were identified experimentally as the main intermediate products in the presence of dissolved oxygen. The availability of dissolved oxygen played a significant role in enhancing the photocatalytic degradation of 4-CP for all the catalysts. (C) 2003 Elsevier B.V. All rights reserved.