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The hydration of Na-saturated Wyoming-type montmorillonite is investigated by Monte Carlo simulations at constant stress in the NP[sub zz]T ensemble and at constant chemical potential in the μVT ensemble, at the sedimentary basin temperature of 353 K and pressure of 625 bar, equivalent to 2–4 km depth. The simulations use procedures established in Chávez-Páez et al. [J. Chem. Phys. 114, 1405 (2001)]. At these conditions, simulations predict a single stable form of 1,2-water layer Na–montmorillonite, containing 164.38 mg/g or 53.37 molecules/layer of adsorbed water and having a spacing of 12.72 Å. The corresponding density is 0.32 g/ml. Sodium ions are coordinated with six molecules of water separated 2.30–2.33 Å. Water molecules are closer to the central interlayer plane and the spacing is larger than that at 300 K and 1 bar. The interlayer configuration consists of two symmetrical layers of oriented water molecules 1.038 Å from the central plane, with the hydrogen atoms in two outermost layers, 3.826 Å apart, and the sodium ions on the central plane located between the water layers. The interlayer configuration can be considered to be a stable two-layer intermediate between the one- and two-layer hydrates. Our simulations do not predict formation of other hydrates of Na–montmorillonite at 353 K and 615 bar. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]