Molecular structure in the interlayers of sodium Wyoming montmorillonite with one, two, or three water monolayers was investigated by Monte Carlo and molecular dynamics simulation, based on tested water-water, water-counterion, clay-water, and clay-counterion potential unctions. Calculated layer spacings and thermodynamic properties, as well as interlayer water configurations and interlayer-species self-diffusion coefficients, were in good agreement with available experimental data. Inner-sphere surface complexes of Na+ with tetrahedral charge sites persisted in all hydrates, whereas the outer-sphere surface complexes with octahedral charge sites found in the one-layer hydrate dissociated partially into an incipient diffuse layer in the two- and three-layer hydrates. Even in the three-layer hydrate, interlayer water molecules did not replicate their structure or mobility in bulk aqueous solution. Improved diffraction and spectroscopic data on interlayer water are needed to guide future simulation studies.