To clarify conflicting reports on the composition and stability of H2O and D2O adsorbates on Ru(0001), we have conducted correlated experiments in two laboratories using X-ray photoelectron spectroscopy and thermal desorption spectroscopy, without and after controlled electron irradiation. Both species form molecular wetting layers; D2O stays molecular to desorption while H2O undergoes a thermal conversion that has been interpreted as partial dissociation. Electron impact leads to this conversion for both H2O and D2O with very high cross-sections (in the high 10(-16) cm(2) range at 100-200 eV) and low threshold energies. These effects may explain discrepancies in recent reports. (c) 2005 Elsevier B.V. All rights reserved.