In the present work, the initial stages of the photocatalytic oxidation of toluene over two commercial TiO2 powders were investigated at the molecular level by ESR spectroscopy. UV-irradiation of the TiO2 samples, which present different phase composition and surface area, gave rise to several oxygenated radicals like O center dot-, O-2(center dot-) and O-3(center dot-), as well as Ti3+ centers. The proportion of these species generated depends on the gaseous environment (vacuum or oxygen), and the structural and morphological characteristic of the TiO2 samples. In contrast, co-adsorption of toluene and oxygen on any of the TiO2 samples studied yields upon UV illumination slightly different ESR signals, which have been assigned to the formation of benzylperoxy radicals, Ph-CH2OO center dot, adsorbed on the semiconductor. Such species are only detected when TiO2 is exposed to tolueneoxygen mixtures enriched in the organic, and under these conditions neither of the other oxygenated radicals is formed. The reasons for this behavior are discussed on the basis of the mechanism for the photocatalytic oxidation of toluene. (c) 2007 Elsevier B.V. All rights reserved.