For improving the thermal stability of organic mineral fertilizers (OMF) organic and inorganic substances are used. Urea is the substance having the best indiced in this respect. The researchers in this field work on finding out cheaper, accessible and economically profitable materials. The present article investigate the effect of natural clinoptilolite on the thermal properties whereas its modified cationic form increase the decomposition temperature and speed up the inhibition of urea. Unrefined clinoptilolite after having been used to purification of waste water containing metallic cations (f.e. from mining, dressing or metallurgy), is recommended to be use as a material for obtaining OMF.