The results of a study of the degree of swelling in two types of cation-exchange resins and a paper wood pulp, all in acid form, are presented. The results obtained suggest that two important factors control the extent of swelling in macroreticular polyacids, namely, (1) the porosity of the polymeric network and how hydrophilic the polymeric walls are (capillary term) and (2) the degree of acidicity of the acid groups attached to the polymeric network (osmotic term). These two contributions to swelling are shown to be quantitatively estimated from the minimization of swelling in media of increased sodium chloride content.