The exchange equilibria of several amino acids, with particular emphasis on those displaying two basic groups, on a strong-acid cation exchange resin (Amberlite IR120) are investigated. The behavior of the experimental data is described through a mathematical model which takes into account the dissociation equilibria of amino acids in aqueous solution, the electroneutrality condition, and an ion-exchange isotherm recently proposed in the literature. In the latter, by assuming ideal behavior for both the solution and the solid phase, the exchange process is treated on the basis of the mass action law, while the existence of a distribution of two types of functional groups with different energies is assumed in order to describe the resin heterogeneity. The developed model is then used in order to satisfactorily correlate binary uptake data and to predict with good accuracy the uptake of amino acids in multicomponent systems.