In this paper capacities of adsorption of nitrogen onto two clays at 77 K are compared and analyzed. One system is a montmorillonite of Camp-Berteau saturated by potassium cation and the second one is an hydrotalcite saturated by carbonate anion. Adsorbed amounts have been obtained using a volumetric technique. So it is shown that the smectite adsorbed more than the hydrotalcite at the same relative pressure of nitrogen. The modelling of experimental data by various theories of adsorption as de Brunauer, Emmett et Teller (BET) and t-method are used to calculate the specific surface and to have access to the porosity of these materials at 77 K. Then it is shown that the adsorption of nitrogen on hydrotalcite is similar as that on the surface of oxides Like the reference. Two endothermic peaks are observed on DTA curves, the first peak may be attribueted to dehydration and the second one to dehydroxylation in the case of smectite and to decomposition of carbonate ions in the case of hydrotalcite.