Effects of water on various acid-catalyzed reactions have been studied mainly using MoO3/ZrO2. While the reaction rate of gas-phase esterification of acetic acid with ethanol and the dehydration of 2-butanol over SiO2-Al2O3 decreased significantly by the addition of water vapor in the feed gas, these reactions over MoO3/ZrO2 were found to be greatly enhanced by the presence of water. The enhancement by water was reversible and was remarkable when MoO3/ZrO2 was calcined at a temperature range of 773-1073 K. IR spectroscopy of H2O adsorbed on MoO3/ZrO2 suggests that the acceleration effect of water would be due to the generation of acidic sites from water coordinated to Mo species on the surface MoO3/ZrO2.