Density functional theory calculations have been applied to investigate the adsorption geometry of water overlayers on the NaCl(1 0 0) surface in the monolayer regime. Competition between H-H intermolecular repulsion and the attraction of the polar molecules to the surface ions results in the most stable structure having a 2 x 1 adsorption symmetry with an adsorption energy of 415 meV. Overlayers of 1 x 1 symmetry, as observed in experiment, have slightly lower adsorption energies. The layers are also unstable with respect to rotation of individual molecules. Multiple hydrogens/oxygens interacting with a single substrate ion can pull that ion out of the surface, although the examples considered are energetically very unfavourable. Overlayers of 1 x 1 symmetry with a coverage of one water molecule per NaCl do not have a high enough adsorption energy to wet the surface. (C) 2007 Elsevier B.V. All rights reserved.