In the study of CO poisoning of the platinum-based hydrogen anode in the polymer electrolyte fuel cell, a key issue that has eluded our understanding is the interactions of CO adsorbed on Pt surfaces and solvent H2O. Our density-functional theory calculations reveal a new interpretation of the adsorbed state of CO and its interaction with water under electrochemical conditions, which rationalizes the observed quantitative relationship between infrared intensities for adsorbed bridging CO(bridge) and water exhibiting a high-frequency O-H stretch (ca. 3650 cm (1)). The theoretical modeling indicates that the observed feature is due to a water molecule firmly hydrogen-bonded to CO(bridge). (C) 2009 Elsevier B. V. All rights reserved.