Water addition to a reaction solvent accelerates the catalytic oxidation of phenol with hydrogen peroxide over titanosilicalite-1 (TS-1). The amount of produced dihydroxybenzene (hydroquinone and catechol) increases from 4.33 to 6.58 mmol with increasing the water concentration in methanol solvent from 14.4 up to 57.2% by weight. Density functional theory (DFIF) study shows that the coordination of a hydrogen peroxide to a Ti(OSiH3)(4) cluster, which is a model of the reaction center of TS-1, is stabilized by the coordination of a water or a methanol. The coordination stabilization energy of Ti(OSiH3)(4)-H2O2-H2O, -48.1 kJ/mol, is greater than that of both Ti(OSiH3)(4)-H2O2-15.8 kJ/mol, and Ti(OSiH3)(4)-H2O2-CH3OH, -44.1 kJ/mol. These results suggest that the addition of water into the solvent increases the amount of coordinated hydrogen peroxide to the Ti center, which is considered to be an active species, or its precursor, of oxidation, and consequently accelerates the reaction rate.