In this paper we present an analysis of the surface of an Algerian montmorillonite. We use two classical methods and we develop two new methods of following the variation of the specific surface area by thermogravimetry and vapor adsorption isotherm, immersion, and wetting calorimetry measurements. This analysis is used to interpret the high value of the enthalpy of immersion In water obtained with swelling clays. This high value seems due to interactions between exchange cations and interfacial water during swelling, which furnish some heat. This heat is estimated following the model we propose of variation of the surface area. The value obtained is on the order of magnitude of an electrostatic interaction,