The generation of hydroxyl ( • OH) radicals produced by irradiation of aqueous TiO 2 suspensions was investigated by electron paramagnetic resonance (EPR) and product analysis employing a stable free nitroxide radical (3-carboxyproxyl) as a spin trap of • OH. Product analysis demonstrated that the attack of • OH on 3-carboxyproxyl produces a diamagnetic product (proxyl-NH) with a trapping efficiency of ca. 80%. It could also be shown that a small amount of the nitroxides is reduced by conducting band electrons. The rate of formation of • OH could be determined by monitoring the time dependence of the decrease in the concentration of 3-carboxyproxyl monitored by EPR spectroscopy or the time dependence of the increase in the concentration of proxyl-NH monitored by gas chromatography analysis. The rate of formation of • OH serves as a mechanistic parameter to investigate the mechanism of formation of this reactive species by the photoexcitation of TiO 2 . The dependence of the rate of formation of • OH was shown to be a linear function of light intensity at low intensities. It was observed that molecular oxygen, a good scavenger of conduction band electrons, only slightly influences the rate of formation of • OH and supports the production of • OH from photogenerated holes. Addition of selected anions to the photolysis mixtures results in a diminished rate of disappearance of the spin trap as the result of a competition between the spin trap and the anion for • OH. From a SternVolmer analysis of the data, the rate constant for the reaction of hydroxyl radicals with the anions was determined.