Abstract The catalytic effects of polymeric sulfonic acids in ester hydrolysis in aqueous acetone were inferior to that of hydrochloric acid. However, the hydrolysis rate was accelerated by increasing the water content of acetone-water mixtures, and, in a water solution, polymeric sulfonic acids were much more effective catalysts. Furthermore, the catalytic efficiency of polymeric sulfonic acid in ester hydrolysis in water increased with the length of the ester molecule. The polymeric sulfonic acid with a strong hydrophobic character was more effective; 69% sulfonated polystyrene-sulfonic acid, for example, hydrolyzed methyl hydrogen azelate in water over five times more rapidly than hydrochloric acid. The specificity thus obtained suggests that, in the hydrolysis of electrically-neutral esters in water, ester molecules are concentrated in the neighborhood of the polymer by the hydrophobic interaction between catalyst and substrate. Besides, in the hydrolysis of aliphatic esters catalyzed by polyvinylsulfonic acid in 50% aqueous acetone, there was a linear relationship between the reciprocal of the rate constant and the intrinsic viscosity of the polymer, while in the case of polystyrenesulfonic acid hardly such any relationship was found. These remarkably different behaviors of the two kinds of polymer catalysts should be attributed to the difference in the expansion of the polymer chain in solutions.