In light of the controversial discussion of the interfacial structure of water on metal surfaces, adsorption of DO on Ru(001) is investigated using sum-frequency generation vibrational spectroscopy. In contrast to a recent theoretical study claiming partial dissociation of the first bilayer of water on Ru(001) [Science 295 (2002) 99], clear spectroscopic evidence is found that the adsorbed molecules remain intact. In the context of all experimental data available, a bilayer structure where every second water molecule exhibits a hydrogen metal bond with a comparably large, proton-disordered unit cell seems most plausible. (C) 2003 Elsevier B.V. All rights reserved.