The dependence of the crystalline swelling and hydration properties of clay minerals on interlayer ion size and charge was investigated using molecular dynamics computer simulations of Na-, Cs-, and Sr-substituted montmorillonites. For all clays studied, layer spacings measured as a function of water content exhibited plateaus at the one-layer hydrate spacing. Calculated immersion energies exhibited minima for integer-layer hydrates up through the three-layer hydrate, with apparent global minima identified with the one-layer hydrate for Cs-montmorillonite and with the two-layer hydrates for the Na- and Sr-montmorillonites. In addition for Sr-montmorillonite, layer spacings jumped discontinuously between one-layer and two-layer separations and showed a second plateau at the two-layer hydrate spacing. The immersion energy curve for Sr-montmorillonite showed similar discontinuities. These results provide evidence for a constant water content swelling transition between one-layer and two-layer spacings. This transition was further characterized using pressure versus layer-spacing isotherms. The isotherms showed a loop structure with peaks that correlated with the coordination number of the interlayer ion. Integration of the Sr-clay isotherms yielded Gibbs energy curves with minima at both one-layer and two-layer spacings. The results indicate that the mechanism of swelling and hydration depends upon the interlayer;ion charge. For monovalent ions studied, swelling and hydration were coincidental, with water,occupying all available interlayer volume. In contrast, expansion of Sr-montmorillonite, driven by formation of a first and partial second hydration shell for Sr2+, occurred initially without the filling of the interlayer volume. Trapping in metastable expanded states was also observed for Sr-montmorillonite and identified as a possible mechanism for layer-spacing hysteresis.