Abstract We present ab-initio periodic Hartree–Fock calculations of water molecules on TiO 2 and MgO. On MgO, H 2O does not dissociate and is adsorbed parallel to the surface. The main interaction concerns the Mg from the surface and the p pair of the water. At saturation, adsorbate–adsorbate interactions favor the formation of an ice monolayer. On TiO 2 the water molecule is strongly adsorbed, dissociating to generate surface hydroxyl groups. The reactivities of the two surfaces are compared and explained in terms of hard and soft acid and base theory.