One-electron oxidation of 4-methyl-p-tolylsulfide (NITS) and 4-methylthiophenylmethanol (MTPM) adsorbed on the surface of TiO2 powder slurried in acetonitrile has been investigated by the time-resolved diffuse reflectance spectroscopy. Relatively high concentration of the adsorbed MTPM determined by UV absorption spectral measurements clearly indicates that the -OH group of MTPM plays an important role in adsorbing on the surface of TiO2 particles. The initial concentration of the radical cations of substrates generated from one-electron oxidation with valence band holes or trapped hydroxyl radical significantly depended on the substrate concentration adsorbed on the TiO2 surface. (C) 2003 Elsevier B.V. All rights reserved.