Sodium montmorillonite (Na-M) was investigated as an energy storage material. The study was conducted through the adsorption isotherms, infrared (IR) spectroscopic and differential thermal analysis (DTA). The clay systems were subjected to different preheating temperature. 125, 160, and 200 degreesC, before subjecting the clay to the adsorption process carried out. The adsorptive capacities of Na-M ranged between 0.12 and 0.48 g H2O/g of dry clay, depending on the preheating conditions of the clay. The stored energy of Na-M 781 cal/g when the clay preheated to 200 degreesC, at which the clay has lost all the water molecules that could be evaporated, and become, therefore, able to adsorb the maximum amount of water vapor allowed. Na-M, when appropriately set, could be considered as an energy storage material.