Adsorption isotherms for water vapor, basal spacing, and immersion heats and water desorption heats of Li+, Na+, Mg2+, Ca2+, Cu2+, and Fe3+ montmorillonite are measured at various relative humidities (r.h.). The amount of water adsorbed as a function of r.h. is found to increase gradually, whereas basal spacing increases and the adsorption heat decreases in steps. The water desorption heat also decreases in steps. The entropy of adsorbed water appears to be negative with respect to the entropy of liquid water. A theoretical model is proposed to describe the hydration process of Li+, Na+, Mg2+, Ca2+, Cu2+, and Fe3+ montmorillonites. The experimental adsorption heats are found to have a direct relationship with the sum of the hydration energy of the cations plus expansion energy.