Adsorbed amounts and heats of the H2O(g)/Na-montmorillonite (Camp-Berteau, Morocco) system have been obtained using gravimetric and calorimetric techniques at 298 K as a function of water vapour relative pressure. The study was completed by the adsorption of nitrogen at 77 K using a volumetric technique. BET theory, the t method and the FHH model were employed to analyse the results. These models led to values of the micropore volume (0.7 mmol g(-1)), the external surface area (110 m(2) g(-1)) and the internal surface area (300 m(2) g(-1)). The hydration delay of this Na-smectite is reflected by the decrease of the parameter s (1.5) of the FHH equation. The variations of heats of adsorption during hydration account for successive steps of swelling and cation migration, in agreement with the different models.