Photocatalytic oxidation in aqueous phase of imazapyr, a systemic herbicide characterized by its high persistence and mobility in soils, has been investigated using two different titania photocatalysts: commercial Degussa P-25 and industrial Millennium PC-500. Despite potentially better textural and crystalline properties of PC-500, its photocatalytic activity is smaller than that of P-25 for that reaction. A less efficiency of PC-500 powders absorbing the irradiated photons, as well as a low substrate adsorption capacity, has been attributed as the main responsible for such behaviour. Variables such as concentration of imazapyr, amount of catalyst and the physical state of TiO2 (as a powder or deposited on an Ahlstrom paper) have been studied to elucidate their influence on the efficiency of the photocatalyzed degradation of imazapyr. It has been determined that both adsorption capacities of the catalyst and the reaction rates are strongly affected by the above mentioned parameters. They are even able to modify both the mechanism and kinetic order of the process, by affecting the interfacial substrate-TiO2 interactions. © 2005 Elsevier B.V. All rights reserved.