The Avnir-Jaroniec method has been applied in the evaluation of the fractal dimension of a montmorillonite pillared with alumina and silica-alumina. The nitrogen adsorption data of the solids at 77 K were used in this method and in the relative pressure range 10(-6) to 0.99. The fractal dimensions depend on the ranges of relative pressures used to apply the Avnir-Jaroniec equation. The interval of relative pressures between 0.08 and 0.2 is proposed to evaluate the fractal dimensions of these solids.