We investigated the detailed mechanism of the relationship between the H+ concentration in the bulk phase and the activity of protonic acid sites on the catalytic surface. The theory clearly shows that the acidity of the catalytic surface is strongly affected by the change of the ion product in the bulk phase water. This phenomenon was apparent with other kinds of metal oxide catalysts and is largely caused by the change in the surface charge of the catalyst. In fact, through the kinetic analysis of the catalytic hydration of propylene with a TiO2 catalyst in sub- and supercritical water, the reaction rate of the hydration could be expressed as a function of both the reaction temperature and ion product of water.