Photocatalytic activity of TiO2 particles embedded in mesoporous silica (TiO2@MPS), prepared by surfactant-templating method with a TiO2 colloidal suspension, has been studied in water with molecular oxygen. Photocatalytic reactions of several kinds of aromatic molecules with TiO2@MPS reveal that less polar substrates show high reactivity, while polar substrates show much lower reactivity. Electron spin resonance analysis with spin-trapping reagents reveals that less polar substrates diffuse easily into the pore of the catalysts and react efficiently with short-lived hydroxyl ((OH)-O-center dot) radicals formed at the surface of inner TiO2 particles, resulting in a high reactivity. in contrast, polar substrates cannot diffuse into the pores and hence, shows a low reactivity. The photocatalytic activity of TiO2@MPS driven by the substrate polarity is applicable to selective transformation of a less polar reactant to a polar product. such as hydroxylation of benzene to phenol with high selectivity (>69%). (C) 2009 Elsevier Inc. All rights reserved.