A model based on the mass action law for the prediction of multicomponent ion exchange equilibria is developed. Ideal behavior for both the solution and the solid phase and the existence of a distribution of functional groups with different adsorption energies (and then equilibrium constant) are assumed. The parametric behavior of the model is analyzed by considering the ion exchange equilibria between both homovalent and heterovalent ions. The reliability of the model, which is characterized by two parameters for each binary system, is tested by comparison with experimental results for inorganic species as well as for several amino acids on strong acid cation exchange resin. It is shown that ion exchange in system involving NC counterions can be predicted from the knowledge of the experimental behavior of only (NC-1) binary systems, thus in agreement with the so called triangle rule.