The adsorption of organic acids on a new weakly basic ion exchanger, highly porous polyaminated chitosan (Chitopearl CCS), which has the primary amino group of chitosan and the primary, secondary, and tertiary amino groups of poly(ethylene imine), appeared feasible technically. Three different organic acids, acetic acid (R＇-COOH), malic acid (R "-(COOH)(2)) and citric acid (R＇＇＇-(COOH)(3)) were used in this experimental study. These organic acids were adsorbed on the resin by an acid/base neutralization reaction. The adsorption isotherms were independent of the initial concentrations of organic acids. The theoretical equations for the adsorption isotherms were derived by considering the dissociation of organic acids in the solution and the adsorption on each functional group and by applying the mass action law. They correlated the experimental adsorption isotherms and titration curves well. Chitopearl CCS was observed to be a feasible medium for the adsorption of organic acids. Especially in high pH region (low concentration region), Chitopearl CCS could adsorb more organic acids than a commercial ion exchanger, DIAION WA30.