We carried out a catalytic hydration of propylene with MoO3/Al2O3 in sub- and supercritical water. MoO3/Al2O3 catalyst promoted hydration of propylene to form 2-propanol as a sole product and exhibited stronger activity than Al2O3, which is known to function as an acid catalyst. It was also experimentally found that the reaction temperature near the critical temperature of water gave a maximum conversion of propylene. The rate of this catalytic hydration could be expressed as a function of both the reaction temperature and the ion product of water, implying that the catalytic activity of the acid sites in MoO3/Al2O3 is strongly affected by the concentration of H+ in the bulk phase.