Abstract: The interaction between “unhydrated” cations (those not separated from the surface by a solvent sheath) in hole positions and the montmorillonite surface was analyzed theoretically by considering the main contributions to potential energy from the coulombic, hydration, van der Waals, induced dipole, and repulsive energies. The effects on these energy terms of the distance between the cation and the plane of basal oxygens, h, and of the angle of rotation of the silica tetrahedra, θ, were investigated. Increase of θ with h constant increases the absolute values of all but one of the energy terms. The hydration energy is an exception because it is probably independent of θ. For a small cation, the increase in attraction is greater than the increase in repulsion when the value of θ is sufficiently small. As θ increases, the repulsive energy becomes more and more dominant until a minimum potential energy is reached. For large cations, this minimum can occur only above a certain value of h. Thus, the values of the potential energy minimum and of θ at this minimum depend on the cation under study as well as on h. Since the concentration of unhydrated cations in a montmorillonite-water system increases with decreasing water content, it is concluded that θ increases during the drying of homogeneous montmorillonite-water suspensions. This provides a partial explanation for the changes in b-dimension with water content observed by other investigators.