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-SiO 2 composite containing 20wt% Aciplex has a surface area of 1.3m 2 g - 1 and possesses an ion-exchanged capacity of 0.46meq.g - 1 after pretreatment at 423K, which is higher than that of 13wt% Nafion-SiO 2 (0.12meq.g - 1 ). The acid strengths estimated from an initial heat of adsorption of NH 3 were similar for these polymer resin composites. It was found that the Aciplex-SiO 2 was more active than typical solid acids such as Cs 2 . 5 H 0 . 5 PW 1 2 O 4 0 , H-ZSM-5, and SO 4 2 - /ZrO 2 for hydrolysis of ethyl acetate in excess water and esterification of acrylic acid with 1-butanol, while it was less active than Cs 2 . 5 H 0 . 5 PW 1 2 O 4 0 for N-alkylation of acrylonitrile with 1-adamantanol and solid-solid hydrolysis of 2-naphthyl acetate. The Aciplex-SiO 2 was superior in activity to Nafion-SiO 2 for all the above reactions and in thermal stability. These results indicate that Aciplex-SiO 2 is a promising solid acid catalyst for reactions involving liquid phase water.