The swelling of clay minerals on contact with an aqueous solution can produce strong adverse effects in the exploration and production of gas and oil. Molecular dynamics and Monte Carlo simulations were used to study the mechanism of swelling of sodium-montmorillonite. The simulations showed that the abundant clay mineral has four stable states at basal spacings of 9.7, 12.0, 15.5, and 18.3 angstroms, respectively. The amount of swelling and the locations of the stable states of sodium-montmorillonite are in good quantitative agreement with the experimental data.