A model based on the mass action law for the prediction of multicomponent ion exchange equilibria is developed by assuming 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 constants). The reliability of the model, which is characterized by two parameters for each binary system, is tested by comparison with a remarkably large set of literature experimental data. It is shown that ion exchange equilibria in systems involving a total number of N-c counterions can be predicted from the knowledge of the experimental behavior of only (N-c - 1) binary systems, thus in agreement with the so-called triangle rule. This greatly reduces the experimental and modeling effort needed to develop a multicomponent model with respect to other approaches which require the investigation of all possible pairs of counterions present in the system, i.e., N-c(N-c - 1)/2 binary systems.