Hydroxyl radical species are considered to be responsible for many oxidation pathways of chemical compounds initiated by advanced oxidation technologies and particularly in heterogeneous photocatalytic processes. However, not many attempts have been made to quantify the generation rate of these species for newly developed photocatalysts, especially for the large number of supported photocatalysts synthesized during the last years. This work focuses on the evaluation of the photonic efficiency for the hydroxyl radical generation on several silica-supported TiO2 photocatalysts recently developed. The study has been carried out by using methanol as hydroxyl radical scavenger. The influence of the mesoporous structure of the support and of its titania loading on the photonic efficiency of the materials has been analyzed. The importance of diffusional restrictions within the porous structure of the support has also been investigated by using a larger hydroxyl radical scavenger molecule, namely n-butanol. (c) 2005 Elsevier B.V. All rights reserved.