Adsorption of molecules of different sizes on montmorillonite and on hydroxy-Al montmorillonite was studied to examine the effect of sorbate size on the experimental value of the surface area of the clays. For each clay the number of adsorbate units at the adsorption maximum per gram of clay produces a single hyperbolic curve, when plotted as a function of the area occupied by adsorbate, in the range of areas of 22-180 Angstrom(2) per sorbate. The equations of the curves are: N A = 657.7 m(2)/g for the clay and N A = 202.8 A(0.27) for the hydroxy-Al montmorillonite. A dependence of the surface area of the hydroxy-iu montmorillonite on adsorbate size was observed and was attributed to surface irregularities produced by hydroxy-Al interlayering.