The structure of zeolite-type materials used in hydrogen evolution from water is described. The formula of these porous crystalline solids based essentially on tetrahedral networks which include channels and cavities is: M-m/z[mAlO(2) . nSiO(2)] . qH(2)O. Mordenite is one of the siliceous zeolites and a typical formula of a synthetic mordenite is: M2/zO . [Al2O3 . (9-10)SiO2] . 6H(2)O. The spatial formula of modified zeolites is discussed in connection with the ability of some of them to generate hydrogen from hot steam after their vacuum activation.