Na+/K+ equilibrium ion exchange isotherms on Amberlite IR-120 (known as a strong acid resin) in pure methanol, ethanol, 2-propanol, 1-butanol, and 1-penthanol and in mixtures of methanol + water, ethanol + water, 2-propanol + water, 1-butanol + methanol, and 1-penthanol + methanol at 313 K have been determined. Experimental equilibrium data have been satisfactorily correlated using the homogeneous mass action law model (LAM). This model assumes nonideal behavior for both the solution and the solid phase. Wilson and Debye-Huckel equations have been used to calculate activity coefficients in the solid and liquid phase, respectively. It has been observed that the dielectric constant of the solvent or the mixture exerts a strong influence not only on the useful capacity of the resin but also on the equilibrium behavior of the system.