Water adsorption on Ru(0 0 0 1) has been examined to throw light on the controversy surrounding whether water adsorbs intact or as a partially dissociated overlayer. Below 140 K H2O/D2O adsorption produces overlayers which show vibrational spectra characteristic of an intact water bilayer. Above 150 K H2O dissociates to form a mixed (OH/H2O/H) overlayer, Suppressing the molecular H2O stretching and scissors modes seen in RAIRS of water bilayers to leave only out of plane bending modes. Dissociation of H2O to form the mixed overlayer competes with H2O desorption and this branching can be enhanced or suppressed by pre-adsorbing O or H. (C) 2004 Elsevier B.V. All rights reserved.