TiO2 photocatalytic one-electron oxidation of 4-methyl-p-tolyl sulfide (NITS), 4-(methylthio)phenyl methanol (MTPM), 4-(methylthio)benzoic acid (MTBA), and 4-(methylthio)phenyl acetic acid (MTPA) adsorbed on the surface of TiO2 powder slurried in acetonitrile (CH3CN) has been investigated by time-resolved diffuse reflectance spectroscopy. As compared to NITS, high concentrations of the adsorbed MTPM, MTBA, and MTPA determined by UV absorption spectral measurements clearly indicate that -OH and -COOH groups play an important role in being adsorbed on the surface of TiO2 particles. From Langmuir-type adsorption/ desorption plots, the apparent equilibrium constants (K-ad) of 20, 60, 2.3 x 101, and 1.1 X 10(4) M-1 were determined for NITS, MTPM, MTBA, and MTPA, respectively. The difference in the Kad values clearly indicates that -COOH and -CH2COOH groups are strongly adsorbed on the TiO2 surface compared with -CH2OH and -CH3 groups. The initial concentration of the radical cations generated from oxidation reactions with holes significantly depended on the concentration of substrate adsorbed on the TiO2 surface. The relationship between the amount of adsorbates and the initial intensity was discussed in terms of Marcus theory. The decay kinetics for transient signals of the radical cations was also discussed.