For efficient and adequate recovery or removal of ions by ion exchange, it is important to know the properties of ion-exchange resins quantitatively. The exchange properties of Amberlite IRC-50, a macroreticular-type weak acid cation-exchange resin, were measured by titration. The properties were modeled by considering ion-exchange sites with different reactivities (polyfunctionality) and suppression of ion exchange with the progress of the reaction due to repulsive lateral interactions between adsorbed ions. It was found that there are two types of sites with similar ion-exchange intensities but different lateral interactions. The different properties of the ion-exchange sites were ascribed to the micro- and macropore environments of the resin. The adsorption affinity of alkali metal ions for the two types of sites is discussed in terms of the hydrated ion sizes and the complex formation with carboxyl group sites.