The single-parameter model for binary ion exchange equilibria is applied to the prediction of ternary and quaternary equilibria. This model is based on a statistical thermodynamic approach to exchanger-phase behavior, coupled with the Pitzer model for activity coefficients in aqueous solution and an explicit description of ion association phenomena. Exchange between alkali metal and alkaline earth cations on commercially available strong acid resins is investigated in the presence of a variety of nonexchanging anions. All systems can be predicted acceptably using parameters fitted to binary chloride-based systems. Model predictions from these parameters are highly accurate in many cases, while those systems that are not predicted as accurately from binary data are able to be described by recalculating one parameter using the ternary or quaternary data. This model fits all criteria required to be met by any description of ion exchange equilibria, and provides a simple yet highly effective approach to the description of equilibria in "real-world" ion exchange systems.