Various acid-catalyzed reactions, such as hydrolysis of ethyl acetate, esterification of acetic acid with ethanol, dehydration of 2-butanol, isomerization of 1-butene, and skeletal isomerization of n-butane, in the presence of Mo–Zr mixed oxides and other solid acids have been studied. MoO₃/ZrO₂ exhibited high activities only for the hydrolysis and esterification reactions, in which these high catalytic activities could not be explained by the acidic property of MoO₃/ZrO₂ alone, as shown by NH₃–temperature programmed desorption (TPD) studies. It was found that the addition of water significantly accelerated the esterification and dehydration in the presence of MoO₃/ZrO₂ (calcined above 973 K), wheras SiO₂–Al₂O₃ was greatly deactivated by water under similar reaction conditions. Furthermore, in the presence of water vapor, the activity of MoO₃/ZrO₂ for the isomerization of 1-butene did not decline, whereas SiO₂–Al₂O₃ became inactive completely. IR measurements revealed that the water adsorbed on MoO₃/ZrO₂ (calcined at 1073 K) gave a sharp peak at 1628 cm⁻¹, and that this peak gradually decreased at 423 K, while ZrO₂ only or MoO₃/ZrO₂ (calcined at 673 K) did exhibit well-defined peaks. Since the adsorption-desorption behavior of water was in good agreement to the catalytic activity for dehydration of 2-butanol, the acceleration by water was presumably a result of the generation of acid sites by the dissociation of water molecules that are coordinated on the Mo species of MoO₃/ZrO₂.