A model is presented to describe binary ion exchange equilibria with the use of a single fitted parameter per binary system. The proposed model incorporates the Pitzer model for calculation of ion activity coefficients in solution, a description of ion association in a concentrated solution, and a statistical thermodynamic treatment of exchange reactions. The behavior of all binary combinations of Ca2+, Mg2+, Na+, K+, and H+ ions in the presence of the Cl- ion as a nonexchanging anion is modeled, and the model fit to all data sets is found to be acceptable. Fitted parameters for each binary system are independent of the solution concentration, and the model is internally consistent. Model results compare favorably with those of other available models, particularly when taking into consideration the fact that only a single fitted parameter per binary system is used. Accurate prediction of the behavior of binary systems by use of data from other binary systems containing each component in the presence of a common "reference component" is also shown to be possible.