We present a model for the bonding of CO adsorbed on alkali-promoted surfaces that accounts for differences in experimental observations between the singly and coadsorbed systems. Theoretical evidence indicates that the interaction between the alkali metal and the CO on the surface is local and dominantly an ionic one, namely, the alkali atom transfers its electron to the CO, resulting in a large Coulombic interaction. This conclusion is supported by the results of generalized valence-bond calculations and qualitatively accounts for the unusual behavior in thermal desorption, electron energy-loss spectroscopy, Auger, and core and valence photoemission in the coadsorbed system.