A strong interaction between H 2O (or D 2O) and CO has been found on Ni(100) using high resolution electron energy loss spectroscopy (HREELS). Without the presence of water, two CO stretching vibrational bands are seen for all coverages below saturation at 135 K, indicating that both bridge and on-top sites are occupied. For a given CO coverage, the addition of water causes a downward shift in CO stretching frequency in two different ways. First, a third CO stretching band is seen at much lower frequency than the first two, i.e. between 1410 and 1660 cm −1. This is assigned to CO in four-fold sites. Relative intensity among the three bands is transferred from the higher to the lower frequency bands with increasing water coverage. A relatively small dose of water can completely remove all detectable intensity from the on-top band, with intensity being transferred to both the bridge band and the four-fold band. All of the intensity can be further moved from the bridge band to the four-fold band, but only when the water coverage is much higher than the CO coverage. In the second type of frequency shift, the positions of the bridge and four-fold bands shift smoothly to lower frequencies as the water coverage increases. This effect is strongest on the four-fold band. On the other hand, all peaks shift slightly to higher frequency with increasing CO coverage. From the point of view of how the CO affects coadsorbed water, the most obvious effect is a change in the water OH stretching region of the spectra. For a constant water coverage, an increase in CO coverage decreases the amount of water-water hydrogen bonding, as manifested in a shift of the OH stretching peak to higher frequency. A very different behavior is seen when the surface is originally saturated with CO. Water adsorption on top of the CO layer is weakened compared to adsorption directly on the nickel surface and no new CO stretching bands or frequency shifts are seen. The overall picture that emerges is of a strong attractive interaction between coadsorbed water and CO in which nearby water molecules on the surface increase the back donation from Ni to the CO antibonding lowest unoccupied molecular orbital. This favors more highly coordinated sites and decreases the CO stretching frequency.