Prior to equilibrium experiments, a clinoptilolite-type Korean natural zeolite was pretreated with HCl, NaOH, and NaCl to improve the ion-exchange capacity for heavy metals. Single- and multi-species equilibrium data of heavy metals such as copper, cadmium, cesium, and lead on treated and untreated natural zeolites were measured experimentally. For single-species equilibrium data, one of the conventional adsorption isotherms, the Sips equation, was used to fit them and then multi-species equilibrium data were predicted by using the loading ratio correlation (LRC), the ideal adsorbed solution (IAS) theory, and the real adsorbed solution (RAS) theory based on the Sips equation. In applying the RAS theory, we used the Wilson equation for fitting activity coefficients of metal ions in the ion-exchanger phase. It was proven that basic adsorption models except the LRC model could describe multi-species ion-exchange equilibrium for heavy metals/natural zeolite systems well.