The effect of sodium dodecyl sulphate (SDS) on the photocatalytic degradation of 2,4,5-trichlorophenol (2,4,5-TCP) in aqueous TiO2 dispersions under irradiation with simulated AM 1 solar light was investigated. A peculiar kinetic behaviour was observed, with increasing inhibition of the substrate degradation as a function of SDS concentration below the critical micellar concentration (cmc), followed by a subsequent faster degradation around this concentration range and a further degradation rate decrease above the eme. The role of surfactant adsorption onto the suspended semiconductor and the disaggregation of TiO2 microparticles seems to be crucial. The complete degradation of the investigated substrate, still achievable in the presence of SDS without relevant changes in the reaction mechanism, was confirmed suggesting the possible application of photocatalysis for the treatment of surfactant-containing wastes. (C) 2004 Elsevier B.V. All rights reserved.