In this basic research-possibly qualifying for practical application-ionic bonds were replaced with covalent bonds in carboxylic cation-exchange resins, taken as examples of crosslinked reactive polymers. To achieve this aim we applied esterification reactions, comprising several modifications. The said reactive polymers were used in the macroporous and gel states and in a part of the study they had previously been dehydrated to contain acid anhydride groups. It was found that the best conversion of the carboxylic to ester groups was similar to80%. The products can be used for example in nucleophilic substitutions, giving rise to a number of derivative crosslinked reactive copolymers. (C) 2002 Elsevier Science B.V. All rights reserved.