Five bentonite samples were laboratory processed to improve their adsorbing properties by means of a treatment with sodium carbonate at three concentrations: 2%, 3% and 4% by weight. The effectiveness of ion exchange between sodium carbonate and bentonite was controlled through the measure of the swelling volume of the bentonites. All the samples (n = 15) were tested for their hygroscopic properties. Adsorption isotherms were measured at 25 degrees C, using desiccators with silica gel, saturated salt solutions and bi-distilled water The data of weight increase of bentonites were also expressed as a function of the time in order to obtain the adsorption kinetics. Through a mathematical equation the specific mass capacity of each sample was calculated as a function of the a(w) value. Moreover as an example, a comparison of isotherms of one bentonite with four different 'sample foods' was made. Results have shown that the hygroscopic properties of bentonites are closely related to their geographical origin and also that it is possible to modulate the use of bentonites as hygrostatic systems. Furthermore, for most samples it has been calculated that the adsorbing velocity is about 25-32% higher than the corresponding desorbing velocity.