In this paper, we present an analysis of the adsorbed amounts and adsorption heats of water vapour of Ca2+ Camp-Berteau montmorillonite. Measurements were obtained at 298 K using thermogravimetry and calorimetry as a function of relative pressure and the study was completed by the adsorption of nitrogen at 77 K. Experimental data were analysed with BET and t-method theories and compared with those of the Na+ sample. A correlation was shown between models and two domains observed on the calorimetric curves. These domains have been attributed to a multilayer equivalent adsorption. However, the determination of the surface area of clays remains a difficulty. So, the areas of interlayer and external surfaces, and the interaggregate microporosity have been estimated from t-plot and BET theories. Exchanged cation effect on the adsorption process is clearly shown (C) 2004 Elsevier B.V All rights reserved.