Heats of ion-exchange of Na-clinoptilolite in 0.25 M; 1 M; 2 M; 4 M; 8 M KCNS and Ca-clinoptilolite in 0.5 M; 1 M; 4 M; 7.2 M NaNO₃ aqueus solutions were measured by differential calorimeter at 30° C. The content of alkali metal cations was determined by flam-photometry of the initial and the ion-exchanged samples. In the case of Ca-Na system, the degree of ion-exchange srongly depends on the concentration, while for the Na-K system this dependence is negligeable. The heats of ion-exchange were measured per gram and recalculated per gram equivalent exchanged cations. In both systems, the maximum of emitted heat was registered for 1 M solutions. The interpretation of the results is based on the theory of ion-hydration in solutions.