A clinoptilolite-type natural zeolite was pretreated by HCl, NaOH, and NaCl solutions to improve the ion-exchange capacities for heavy metal ions such as copper, lead, cadmium, and cesium. The pretreated natural zeolite was experimentally investigated based on chemical analyses, X-ray diffraction, and BET experiments etc. From experimental data, it was shown that the pretreatment with NaCl gave the best ion-exchange capacity for all metal ions encountered. The ion-exchange capacity of the NaCl-treated sample is comparable with that of a commercialized natural zeolite, chabazite. On the other hand, the HCl-treated sample has very low ion-exchange capacities for all metal ions even though it has high specific surface area and total pore volume. It was proven by chemical analyses that a strong acid like HCl can damage the structure of the zeolite by extracting aluminum and iron from their skeletal units.