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 (μVT 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 D1=11.9 Å and D2=14.4 Å. For Wyoming montmorillonite they are D1=12.2 Å and D2=14.7 Å. It is also found that montmorillonites develop one layer of water at D1, and two layers of water at D2. We found that, for all the water contents considered here, the cations are hydrated.