The adsorption of molecules of different sizes, at monolayer coverages, was determined for binary mixtures of montmorillonite and hydroxy-Al montmorillonite in order to test the applicability of the general equation (NA(z) = k) relating the number of sorbate units at monolayer coverage (N) with their areas (A), The effect of the proportions of the components in the mixture on the value of the exponent z was also evaluated. The data obtained confirm the validity of the equation and indicate that the value of z for the mixtures assumes intermediate values between the two z values of the components, depending on their proportions. It is also demonstrated that the z exponent could have a value of unity (a value supposed to indicate surface uniformity) for certain mixtures of sorbents having z values different from unity, i.e., possessing nonuniform surfaces,