The current work deals with the applicability of the fractal version of the Frenkel-Halsey-Hill (FHH) theory of multilayer adsorption. The concept of the standard isotherm is introduced to examine the weakness of this equation when the solid-gas potential controls the adsorption process of gas on a given solid. The results of the determination of the fractal dimension of typical samples of carbon blacks, activated carbon, and aerogels (mixed SiO2-ZrO2 and pure silica) are given. They show that the fractal dimension depends critically of the coverage exponent s for a flat surface of the same chemical nature.