The contribution of the Cu4sp conduction band electrons to the chemisorption bond of CO/Cu(100) has been analyzed based on ab initio wavefunctions for a cluster model of CO at an on-top site of Cu(100). The Cu 4sp electrons make a larger contribution to the bond energy than do the 3d electrons. The importance of the Cu sp electrons suggests that CO might be chemisorbed, albeit weakly, on alkali metal surfaces. Indeed, our results show that the Li-CO chemical bond could lead to CO/Li(100) being stable at about liquid nitrogen temperature. On the contrary, the interaction of CO with Na(100) is much weaker. © 1992.