A new mechanism of CO adsorption at the three-coordinated (3c) Mg sites of MgO is proposed on the basis of a quantum chemical study of the phenomenon of gradual diminution of the 2202 cm(-1) band of the OC Mg-3c species upon increase of the CO pressure above 10(-1) Torr [Coluccia, S., et al. Spectrochim. Acta 1993, 49A, 1289]. The MgO surface is modeled by bare Mg10O10 and Ng(16)O(16) clusters at the HF and DFT (B3LYP) levels of theory. We examine the hypotheses that the intensity decrease may be due to (i) the static dipole-dipole or/and inductive interactions with the CO molecules at the neighboring Mg-4c sites and (ii) the coadsorption of two or three CO molecules at the Mg-3c sites. The computed frequency shifts and binding energies suggest that the discussed phenomenon is due to the coadsorption of two CO molecules at the Mg-3c sites.