The modifications of the microporous volume of zeolites resulting from thermal dealumination have been examined by measuring the changes in the unit cell parameters as well as the N 2 and n-pentane adsorption isotherms. For the cubic HY and orthorhombic H-ZSM-5 zeolites, the differential decrease of the microporous volume per nonframework aluminum (NFA1) can be predicted by assuming NFA1 is concentrated in pores available to N 2 . For the orthorhombic H-mordenite, NFA1 is homogeneously spread in all pores, available or not to N 2 . The Gurvitch rule (volume of adsorbed N 2 =volume of adsorbed pentane) is respected in HY and H-ZSM-5, but not in H-mordenite, irrespective of the degree of dealumination.