The shape selectivity in the liquid-phase hydroxylation of aromatic hydrocarbons with hydrogen peroxide has been studied on two kinds of titanosilicate, the large-pore Ti-M with MOR structure and medium-pore TS-1 with MFI structure, and the liquid-phase diffusion of aromatics into both catalysts was compared in the presence of H2O and H2O2 to clarify the origin of the shape selectivity. The hydroxylation rate on TS-1 decreased monotonically with increasing molecular size in the order benzene>toluene. ethylbenzene>cumene, while Ti-M showed the maximum rate for toluene hydroxylation. The apparent diffusivity of aromatics was lowered roughly by 1 order by adding H2O2 to H2O solvent for both titanosilicates but was not affected for their Ti-free derivatives, mordenite and silicalite-1. The reduction in diffusion rate in the presence of H2O2 was more pronounced for TS-1 and for Ti-rich samples. It is concluded that a bulky Ti peroxo species (Ti-OOH) formed by the interaction of Ti site with H2O2 mainly causes a transition-state shape selectivity in the hydroxylation of bulky aromatics in titanosilicate/H2O2 systems.