The generation of hydroxyl (•OH) radicals produced by irradiation of aqueous TiO2 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 TiO2. The dependence of the rate of formation of •OH was shown to be...