Adsorption isotherms for montmorillonite, illite, green sand, kaolinite, and silica gel unheated and after heating at 600[degree] were obtained gravimetrically by employing a simple desiccator method. These data have been analysed by methods of Brunauer et al. (BET) and Harkins and Jura (HJ) and characterised by parameters. Unheated montmorillonite and illite had widely separated isotherms, which were similar after heating and much higher than poorly sorbing kaolinite. The thickness of water films for these three clays were similar up to 0.5 relative pressure, after which marked differences occurred. Preheating at 600[degree] reduced the specific surface area of montmorillonite by about 80% and little about 30 percent On the existing evidence that this heating eliminates the internal surface of montmorillonite, the isotherms for the internal and external surfaces of montmorillonite and illite both approached 3 molecular layers internally and 6 layers externally at saturation pressure. With certain restrictions the BET and HJ parameters have been related by obtaining a singificant inverse linear relation between the area available per water molecule on the surface, and the relative pressure at mono layer coverage. Hysteresis was found to remain fairly constant in the presence and absence of internal surface for montmorillonite and illite, so that hysteresis appears to be a function of the external surface of these pure clay minerals.