Heterogeneous esterification of poly(2-hydroxyethyl acrylate) (PHEA) gel with acylating agents such as acetic anhydride, a mixture of acetic anhydride and pyridine, and chloroacetic anhydride was carried out under various conditions. The selective esterification of PHEA gel proceeded smoothly in toluene from surface to give a novel core-shell type gel (gel capsule) consisting of a hydrophilic unreacted core and hydrophobic acetylated shell, and finally afford almost quantitative acetylated PHEA gel. The reaction was strongly affected by kinds and concentration of acylating agents. The swelling of the obtained gel capsule was studied. Although the gel capsule obtained from the esterification of PHEA gel with acetic anhydride showed much the same swelling behavior as the original PHEA gel, the shell layer resulting from the esterification of PHEA gel with a mixture of acetic anhydride and pyridine showed a high degree of esterification and worked as a permeability barrier of water. (C) 2004 Elsevier Ltd. All rights reserved.