Time-of-flight neutron diffraction has been used in conjunction with isotopic substitution to obtain high-resolution structural data for clay-water-cation systems, at elevated pressures and temperatures. We have developed a new sample environment, that allows us to study clay-fluid interactions in situ, under hydrostatic fluid pressures of up to 2 kbar, and temperatures of up to 350 °C. These conditions approximate to those encountered in sedimentary basins, at burial depths of up to 12 km. In this paper we present new results for hydrated clays in which the interlayer cations are magnesium or calcium. We find that throughout our experiments these divalent ions are octahedrally hydrated, as they would be in the bulk. However, hydrogen bonding of interlayer water molecules to the clay surfaces disappears as we approach the critical point. Our data support the assertion that interlayer water is denser than the bulk.