The removal of chloride, nitrate, and sulfate ions from wastewaters by a macroporous ion exchange resin is studied through the experimental results obtained for six ion exchange systems, OH-/Cl-, OH-/NO3-, OH-/SO42-, and HCO3-/Cl-, Cl-/NO3, Cl-/SO42-. The results are described through thermodynamic modeling, considering either an ideal or a nonideal behavior of the ionic species in the liquid and solid phases. The nonidealities are determined by the Davies equation and Wilson equations in the liquid and solid phases, respectively. The results show that the resin has a strong affinity for all the target ions, and the order of affinity obtained is OH- < HCO3- < Cl- < NO3- < SO42-. The calculation of the changes in standard Gibbs free energies (Delta G(0)) shows that even though HCO3- has a lower affinity to the resin, it may affect the removal of Cl-, and in the same way that Cl- may affect the removal of NO3- and SO42-. The application of nonidealities in the thermodynamic model leads to an improved fit of the model to the experimental data with average relative deviations below 1.5% except for the OH-/SO42- system. On the other hand, considering ideal or nonideal behaviors has no significant impact on the determination of the selectivity coefficients. The thermodynamic modeling is also compared with the Dubinin-Astakhov adsorption isotherms obtained for the same ion exchange systems. Surprisingly, the latter performs significantly better than the ideal thermodynamic model and nearly as well as the nonideal thermodynamic model.