Surfaces of SiO2-covered titanium dioxide (TiO2) photocatalysts (SiO2-TiO2) were modified with alkylsilyl groups such as hydrocarbon, fluorocarbon, and phenyl groups. Oxidation of propanal, hexanal, nonanal, pentafuluorobenzaldehyde, and benzaldehyde proceeded much more efficiently on the surface-modified SiO2-TiO2 particles than on SiO2-TiO2 photocatalysts without surface modification. In the case of SiO2-TiO2 photocatalysts modified with hydrocarbon chains, one of the most important factors for improvement of the photocatalytic activities of surface-modified TiO2 photocatalysts is the hydrophobic interaction between substrates and alkylsilyl groups introduced onto SiO2-TiO2 photocatalysts. SiO2-TiO2 Modified with phenyl groups is more active in the photocatalytic oxidation of benzaldehyde than SiO2-TiO2-C-6. Furthermore, oxidation of pentafluorobenzaldehyde proceeded more favorably on SiO2-TiO2-C-s(F) chains than on SiO2-TiO2-C-8. These results indicated that the functional groups introduced onto the surface of SiO2-TiO2 play an important role in showing selectivity for photocatalytic oxidation of organic compounds. (c) 2005 Elsevier B.V. All rights reserved.