Adsorption of a strong acid on a new weakly basic ion exchanger, highly porous polyaminated chitosan (PEI-CH), which was fabricated by introducing poly(ethylene imine) (PEI) into highly porous chitosan spherical particles (CRCH), appeared feasible technically. The saturation capacity for adsorption of HCI on PEI-CH was about 1.75 times larger than a commercial weakly basic resin DIAION WA30. HCl was adsorbed on them by an acid/base neutralization reaction. Since DIAION WA30 has a single kind of fixed functional group, the experimental isotherm and titration curve were correlated by the Langmuir equation. As PEI-CH has four different fixed functional groups, the primary ammonium group of chitosan and primary, secondary, and tertiary ammonium groups of PEI, a theoretical equation for the equilibrium was derived by considering the adsorption on each functional group. It correlated the experimental titration curve (equilibrium isotherm) well.