Abstract The adsorption of H 2O on Ru(001) was investigated by photoemission spectroscopy (UPS and XPS), LEED and workfunction measurements in the temperature range 120–300 K. The H 2O coverage was calibrated via O1s core level intensity by comparison with a p(2 × 2)-O layer corresponding to a coverage of 0.5. The O1s spectra of adsorbed H 2O layers always showed a contribution near 531.3 eV binding energy although the main intensity was located at 532.7–533.9 eV depending on coverage. Annealing of adsorbed layers to 173–193 K caused an O1s double peak to appear with binding energies at 531.3 and 532.7 eV. These peaks are assigned to islands of a H 2O bilayer whereby the binding energies 531.3 and 532.7 eV belong to the first and second H 2O layer, respectively. Higher order H 2O layers have an O1s binding energy of at least 533.2 eV. The layer-dependent binding energy of H 2O is suggested to be due to a distance-dependent screening effect of the O1s core hole, aside from the first layer H 2O molecules which are chemically bonded to Ru surface atoms via the oxygen lone pair orbital. The formation of perfect bilayer islands is thermally activated and does not occur during low temperature adsorption at 120 K. Up to 15% of the first bilayer of H 2O molecules are found to dissociate after a slow anneal to 210 K whereas dissociation is negligible for a fast flash.