Monte Carlo grand canonical molecular simulations on the hydration of Na-, K-, and Ca-montmorillonite show that between 333 and 533 K and 300-1300 bar Na-montmorillonite forms stable one-layer hydrates of d(001) spacings 12.64-12.38 angstrom, K-montmorillonite of 12.78-12.59 angstrom, and Ca-montmorillonite of 12.48-12.32 angstrom. A two-layer hydrate of 14.80 angstrom occurs for Na-montmorillonite at 533 K and 1300 bar, for K-montmorillonite of 15.32 angstrom at 533 K and 1300 bar and of 14.74 angstrom at 533 K and 2000 bar, and for Ca-montmorillonite of 13.83 angstrom at 473 K and 1000 bar. Three-layer hydrates may possibly form within these same ranges. Outside of them, one-layer hydrates simulate as the only stable hydrates. In sedimentary basins, the two-layer hydrate of Ca-montmorillonite will locate at 6.7 km depth and those of Na- and K-montmorillonite at 8.7 km depth; above and below these depths, the one-layer hydrates are the stable phases.