The structural and electronic properties of interfacial water and adsorbed CO on platinum and platinum/ruthenium alloy have been studied via density-functional theory calculations to gain insight into the water-adsorbate interaction under electrochemical conditions. The computational simulations reveal a new interpretation for the interaction of adsorbed CO and water at the electrochemical interfaces. The new interaction model rationalizes the observed quantitative relationship between infrared intensities for adsorbed bridging CO and water molecules that impart a high-frequency O-H stretch, ca. 3630-3660 cm(-1) on pure Pt and 3600-3620 cm(-1) on PtRu alloy. The theoretical modeling indicates that the observed feature common to both pure Pt and PtRu alloy surfaces is due to interfacial water molecules firmly hydrogen-bonded to bridging CO. (C) 2010 Elsevier Ltd. All rights reserved.