water adsorption in clays in controlled conditions of RH had received considerable attention in recent years. When the sheets of expandable clays had been balanced with water vapor by increasing of RH, a successive adsorption of water in layers was observed. It was deduced from the shape of the adsorption isotherms and from changes of certain features of the patterns of X -ray diffraction related with basal spacing (Montes-Hernandez and Geraud, 2004; Dios Cancela et al., 1997). Thus, these materials were potentially important to use as a desiccant due to its high adsorption capacity. They could be improved by purification and addition of chemicals such as CaCl2 to obtain desiccant mixtures of higher hygroscopicity and higher stability (Thoruwa et al., 2000). In this work it is studied the effect of purification and modification of a bentonite from Argentina with CaCl2 in humidity retention, in order to evaluate the conditions for improving the desiccant capacity.