A silica composite of a perfluorocarbonsulfonic acid resin, Aciplex, has been used as a solid acid catalyst for a variety of reactions concerning water. The Aciplex-SiO2 composite containing 20 wt% Aciplex has a surface area of 1.3 m(2) g(-1) and possesses an ion-exchanged capacity of 0.46 meq.g(-1) after pretreatment at 423 K, which is higher than that of 13 wt% Nafion-SiO2 (0.12 meq.g(-1)). The acid strengths estimated from an initial heat of adsorption of NH3 were similar for these poly mer resin composites. It was found that the Aciplex-SiO2 was more active than typical solid acids such as Cs2.5H0.5PW12O40, H-ZSM-5, and SO42-/ZrO2 for hydrolysis of ethyl acetate in excess water and esterification of acrylic acid with 1-butanol, while it was less active than Cs2.5H0.5PW12O40 for N-alkylation of acrylonitrile with 1-adamantanol and solid-solid hydrolysis of 2-naphthyl acetate. The Aciplex-SiO2 was superior in activity to Nafion-SiO2 for all the above reactions and in thermal stability. These results indicate that Aciplex-SiO2 is a promising solid acid catalyst for reactions involving liquid phase water.