To improve and extend the properties of organic polymer-supported catalysts, the corresponding functional polymers were grafted on porous silica. Mechanical stability and constant shape, which are independent of solvent nature, and the possibility of modulating polarity to fit that of the reagents are new advantages. After a study of the grafting mechanism to understand the extent of grafting and chain length control, we tried to determine the optimal catalyst structure. Styrene was polymerized in the presence of a trifunctional silanoxy chain transfer agent in various amounts and was reacted with silica and then sulfonated. The acid catalytic properties of the resulting materials were evaluated in three tests of interest and were compared to those of reference materials. Selectivity was excellent. Reactivity was dependent upon catalyst compatibility with the reagents and the presence of water, which alters stability. Reactivity was poor for toluene alkylation, moderate for t-butanol dehydration, and good for ketalization. The specific activity was higher for a high number of short chains than for a low number of long chains at a given polymer to silica ratio.