Density functional molecular cluster calculations have been used to study the adsorption of CO on the alpha -Al2O3-(0001) surface. Substrate and adsorbate geometry modifications, adsorption enthalpies, and adsorbate vibrations are computed. Despite the rather small size of the employed cluster, relaxation phenomena evaluated for the clean surface agree well with experimental measurements and periodic slab calculations and mainly consist of an inward relaxation of the Lewis acid site (L-s(a)). Different adsorbate arrangements, perpendicular and parallel to the surface, have been considered. Among them, the most stable CO chemisorption geometry (Delta H (ads) approximate to -13 kcal/ mel) is that corresponding to the adsorbate perpendicular to the surface, atop L-s(a) and C-down oriented. The C-O stretching frequency' (nuc-o) computed for such an arrangement is 2158 cm(-1), i.e., blue shifted by 44 cm(-1) with respect to the free adsorbate. The lack of experimental evidence pertaining to CO interacting with a well-defined alpha -Al2O3(0001) surface prevents the possibility of a direct check of the computed quantities. Nevertheless, low-temperature IR data for CO on alumina powders (Zecchina, A.; Escalona Platero, E.; Otero Arean, C. J. Catnl. 1987, 107, 244) indicate for the chemisorbed species a Delta nu = 12 cm(-1). The adsorbate-substrate interaction relieves some of the L-s(a) relaxation, even if the L-s(a) electronic structure is only slightly affected upon chemisorption.