Some of advanced oxidation processes (AOP) are characterised by a special chemical feature: the ability to use the high reactivity of *OH radicals in driving oxidation processes. These radicals are suitable for achieving the complete abatement, even including the mineralization of less reactive pollutants. In this study, a photocatalytic process is used to degrade one herbicide of the imidazolinone family, Imazapyr. It was shown to be photodegraded rapidly and extensively in an aqueous solution. The decline of Imazapyr concentration in the solution followed a first-order kinetics. The apparent first order rate constant was found equal to 0.19 min(-1) in distilled water at natural pH 3.8. The smaller activities found at acidic and basic pH were explained by considering the ionisation state of Imazapyr and the charge density of TiO2. The present work dealt with the influence of metal ions like Ni2+ and Cu2+ which are frequently present in agricultural wastewater on the photocatalytic efficiency of TiO2 in the elimination of Imazapyr. A detrimental effect of the presence of metallic species was observed only with samples containing amount of copper and nickel in the presence of TiO2. Several hypotheses were proposed to explain this phenomenon, passivation of TiO2 surface by adsorption Of Cu2O and/or Cu-0, formation of a complex or recombinaison of the e(-)/h(+) pairs. At higher concentrations of metallic species like Cu2+ and Ni2+, a plateau was reached which could be explained by the photo-Fenton like reaction. In an attempt to understand the basic mechanisms of the degradation of Imazapyr in water by TiO2 photocatalysis, we discussed the primary degradation mechanism on the basis of the experimental results together with molecular orbital calculation of frontier electron density and partial charge. (c) 2005 Elsevier B.V. All rights reserved.