Adsorption and desorption isotherms of nitrogen at 77 K, on H-mordenite having various SiO2/Al2O3 ratio have been studied. The isotherms have been treated in the BET, Langmuir, Kiselev, Barrett-Joyner-Halenda, and Dubinin-Raduschkevich equation, determining the effect of the dealumination on texture parameters, as well as the limits of applicability for the BET equations in comparison with the Langmuir and Dubinin-Polanyi theories. The presented data confirmed that the BET equation for calculation of the specific surface area does not apply to solids with most of their pares having diameters smaller than four times the molecular diameter of the adsorbed gas; applying it to zeolites lacks any physical support. The adsorbtion potential and the Dubinin-Raduschkevich maximum adsorbtion volume are more suitable for the characterisation of these microporous systems. The texture parameters exhibit maximum values for the sample with the SiO2/Al2O3 ratio equal to 100.