Nitrogen adsorption isotherms have been carried out on Y zeolites and dealuminated Y zeolites, mordenites and dealuminated mordenites, and ZSM-5 zeolites. The treatment of these isotherms by the Dubinin-Radushkevich method has led to a first estimation of the microporous volume of these zeolites. The t method has been used to determine their external surface and to obtain estimation of the microporous volumes. It is shown that the Dubinin-Radushkevich method overestimates the microporous volume of solids with important external surface. It is also shown that the choice of the standard t function and the part of the V-t curve considered for the Linear fitting may drastically influence the microporous volume and the external surface obtained by the t method. A new way of treating the N-2 adsorption isotherm to establish the microporous volume and the external surface of zeolites is proposed. This method assumes that the experimental adsorption isotherm may be decomposed into two parts corresponding, respectively, to the adsorption on the external surface and to the adsorption in the micropores. It is considered that the BET equation governs the adsorption on the external surface, while the adsorption in the micropores can be described by the Dubinin-Radushkevich equation. The correlation coefficient of the BET curve obtained from the experimental isotherm corrected for the adsorption in the micropores is optimized by adjusting the limiting adsorption volume and the adsorption energy of the Dubinin-Radushkevich equation. The C-BET, the microporous volumes, and the energy of adsorption in the micropores obtained in this way are consistent with the data of the literature.