Unlike Cs2.5H0.5PW12O40, Cs2.5H0.5PW12O40-SiO2 composites combined using (3-aminopropyl)triethoxysilane are entirely insoluble and well sedimentable in reaction mixtures that contain a large excess of water. The catalytic activities for the hydration of olefin and hydrolysis of ester in the presence of excess water were comparable to those Of Cs2.5H0.5PW12O40 and were much higher than those of typical water-tolerant solid acids, such as H-ZSM-5 and Nb2O5.