Chemisorption of a CO molecule on regular and defect sites of the MgO(100) surface has been investigated by means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface. The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A blueshift in the C-O stretching frequency, ωe, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric field for low-coordinated cations. Both the bond strength and the ω shift are largely due to electrostatic effects and not to the formation of a dative σ-bond with the surface. Surface relaxation effects have also been considered. © 1992.