Equilibrium ion-exchange isotherms of H+/Cu2+, H+/Cd2+, and H+/Zn2+ on a strong acid resin, Amberlite IR-120, in an aqueous medium at (283 and 303) K have been determined in order to assess the possibility of using ion exchange to eliminate heavy-metal ions from industrial aqueous liquid streams. The 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 solid phases. Wilson and Pitzer equations have been used to calculate activity coefficients in the resin and liquid phases, respectively. The values of the thermodynamic equilibrium constants show a temperature dependence.