A solid acid, Cs2.5H0.5PW12O40, efficiently catalyzed the reactions in which water participates such as hydrolysis of eater, hydration of alkene, and esterification, in excess water. This acidic salt was far superior in the activity to other oxide catalysts such as H-ZSM-5, Nb2O5, SO42-/ZrO2, and SiO2-Al2O3. When Cs2.5H0.5PW12O40 was separated from the reactant solution and reused five times for the hydrolysis of ethyl acetate repeatedly, the reaction rate was retained more than 90%. These results demonstrates that Cs2.5H0.5PW12O40 is an efficient water-tolerant solid acid.