The effect of exchange cations (Mn : Li , Na , K , N H 4 , Ca2 , Mg2 , Fe3 , and Al3 ) and thermal treatment on the physicochemical properties of smectite was investigated. Before thermal treatment, unheated (25°C) Mn -smectite showed increased d001 spacing upon glycol solvation; after heating at 300°C or above (400°C), d001 spacing upon glycol solvation of Al-, Fe-, Mg-, Li-smectite decreased and reached 10 Å. However, an increase in d001 of Ca2 -, Na - and N H 4 -smectite upon glycol solvation indicated that d001 varied with the nature of exchangeable cation and the temperature of heating. The results generally suggested the following: d001 of homoionic smectite before and after heating at various temperatures depended upon the nature of exchangeable cation. The cation exchangeable capacities (CECs) and the BET surface areas (SBETs) were func- tions of the type of exchangeable cations, since SBETs drop off in the following order: Na -smect > Ca2 -smect >2 -smect > N H -smect > Li -smect > K -smect > Fe3 -smect > Al3 -smect. Finally, we have noted good Mg 4 progress in the exchangeable properties of synthesized clay in comparison with the Na homoionic smectite form, except for K homoionic clay, which have the lowest CECs, SBETs, and consequently, the lowest exchangeable properties.