Four hydration water molecules were observed on a Ru(001) surface by infrared reflection absorption spectroscopy (IRAS). An abnormally low frequency shift from 2145 to 1356 cm(-1) of a CO molecule adsorbed onto a Ru(001) surface was observed after coadsorption of water molecules with CO. A flat and a tilted monomer were found to exist as a singleton species at a low and a high coverages, respectively. The degree of charge transfer from an oxygen lone pair of the monomers to a Ru(001) surface is comparable with that from an alkaline atom on the same surface.