Grand isoshear ensemble Monte Carlo simulations have been used to simulate swelling and hydration thermodynamics in smectite clay minerals. Crystalline or short-range swelling in these layered materials is described in terms of a series of layer-spacing transitions that are thermodynamically analogous to phase transitions. Measured disjoining pressures exhibit oscillations as a function of layer spacing, indicating that swelling occurs in discrete steps. Integration of the disjoining pressures yields swelling free energies that in turn may be decomposed into energetic and entropic components. Results are presented for three Na-smectite clays differing in their layer charge magnitude. Increasing layer charge is found to enhance the stability of more swollen clay states. Analysis of swelling energies and entropies reveals an energetic driving force for swelling, with entropy playing a smaller, compensating role. Results are compared with previous studies of smectite swelling as a function of interlayer ion size and charge. (C) 2004 Elsevier B.V. All rights reserved.