In the present study, various transition metal cationic forms of mordenites were prepared. The results of chemical and thermogravimetric analysis could be used to derived the chemical formulae of these mordenites. The indication gained from thermogravimetric studies both in vacuo and in air is that the high vacuum procedure causes a partial collapse in the mordenite structure and this effect is connected with the degree of reversibility of water loss and water re-uptake by the various mordenite samples. The scanning electron micrographs showed that the exchangeable cations do not affect the microcrystals of these mordenites buut their effect is mainly restricted to the intracrystalline regions and the amorphous character of the microcrystallites.