The photocatalytic transformation of MCPA on TiO2 leads to 4-chloro-2-methylphenol (P-3) as the major intermediate. Some minor intermediates were also identified, namely methylhydroquinone (P-1), 5-chioro-2-hydroxy-3-methyl phenyl acetic acid (P-2), 4-chloro-2-methylanisole (P-5) and 4-chloro-2-methylphenylformate (P-4). The rate of transformation is higher with Degussa P25 than with Millennium PC50, PC100. PC 105 and PC500 used as slurries (1 g l(-1)). There is no apparent relationship between the rate of transformation and the surface area of the catalyst. The immobilization of TiO2 on glass fibres significantly reduces the reaction rate, but it has the important advantage of eliminating the problem of filtration. Except with PC100 and PC500 kinetics obey a first-order law. The photocatalytic transformation is probably due to two different processes involving hydroxyl radicals and positive holes. (C) 2003 Elsevier B.V All rights reserved.