Monte Carlo simulations are performed to investigate the role of the divalent cation Ca on the stability and swelling of montmorillonite. Constant stress simulations (NPzzT ensemble) are used to predict the basal spacing as a function of the water content in the interlayer, and constant chemical potential simulations (mu VT ensemble) are used to find the more stable basal spacings of the clay-water systems. Two model clays are considered in this work, namely, Otay and Wyoming montmorillonites. It is found that the equilibrium basal spacings for Otay montmorillonite are D-1 = 11.9 Angstrom and D-2 = 14.4 Angstrom. For Wyoming montmorillonite they are D-1 = 12.2 Angstrom and D-2 = 14.7 Angstrom. It is also found that montmorillonites develop one layer of water at D-1, and two layers of water at D-2. We found that, for all the water contents considered here, the cations are hydrated.