The photocatalysed degradation of 4-chlorophenoxyacetic acid (4-CPA, 1) has been investigated in aqueous suspensions of titanium dioxide under a variety of conditions. The degradation was studied by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The influence of various parameters such as, different types of titanium dioxide (TiO2) Powders, PH, catalyst and substrate concentrations, and in the presence of electron acceptor such as hydrogen peroxide (H2O2) besides molecular oxygen has been investigated. The effects of these parameters on the degradation rates were found to be significant. The volatile degradation product 4-chlorophenol was analyzed by GC-MS technique and probable pathways for the formation of product has been proposed. (c) 2006 Elsevier B.V. All rights reserved.