In this study, with the intention of determining the primary intermediates, the photocatalytic degradation reaction of 4-chlorophenol (4-CP) which has been accepted as the standard pollutant was modelled. Geometry optimizations of the reactants, the product radicals, the pre-reactive and the transition state complexes were performed for all the possible reaction paths. The DFT calculations were carried out by the hybrid B3LYP functional, which combines HF and Becke exchange terms with the Lee-Yang-Parr correlation functional by using 6-31G* basis set. The solvation effects were computed by using COSMO as the solvation model. Based on the results of the quantum mechanical calculations, the rate constants for all the possible reaction paths were calculated by means of the Transition State Theory. The difference in the rates of reaction paths were explained in terms of the presence of hydrogen bonds in the transition state complexes. The product distribution for the photocatalytic degradation of 4-CP was predicted. The results show that the major intermediates of the photocatalytic degradation of 4-CP are 4-chlorocatechol (4-CC) and hydroquinone (HQ) with [4-CC]>[HQ]. 4-Chlorophenoxyl radical which causes the formation of polyaromatics is not produced in either gas or aqueous phases. (C) 2007 Elsevier B.V. All rights reserved.