The radioactive decontamination of water, soil and other materials requires cheap and effective adsorbents. Artificial zeolites synthesized from an industrial waste (coal fly ash: Na-P1 type zeolite) and a natural material (diatomite: mordenite type zeolite) have a high Cs⁺ adsorptivity in the adsorption experiments using 0.1 g of the zeolite and 50 mL of up to 7.5 mM CsCl. The coexisting cation suppressed the Cs⁺ adsorption onto the zeolites, and the effect of the suppression was in the order, K⁺ > Na⁺ > Ca²⁺ > Mg²⁺. A thermodynamic analysis proved that the Cs⁺ adsorption onto the two zeolites was exothermic favoring a lower temperature. The artificial mordenite showed a greater Cs⁺ adsorption strength, higher distribution coefficient and lower ΔG°, especially at low Cs⁺ concentrations. Adsorption isotherm analysis by the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models showed a greater Cs⁺ adsorption selectivity for the artificial mordenite even at a low pH.