The NH4-forms of natural stilbite, clinoptilolite and heulandite were prepared and the thermostability compared with the parent mineral and investigated by X-ray, thermoanalytical and infrared methods. The acidity of the H-forms was determined by adsorption of basic molecules and by measuring the dehydration rate of isopropanol. In contrast to the mineral, the H-form of stilbite and heulandite shows no phase transformations and is thermally the most stable form. Dehydroxylation is accompanied by a gradual loss in crystallinity and the pure dehydroxylated form does not exist. H-clinoptilolite shows a slightly higher thermostability. The hydroxyl spectra, acidity and catalytic activity of H-clinoptilolite and H-stilbite show only minor differences.