The bonding properties of CO molecules adsorbed at a stepped MgO surface have been studied by means of cluster models and gradient-corrected DFT calculations. At a step site CO interacts with both a Mg-4c and a Mg-5c ion forming a relatively strong bond, 0.2 eV, but with very small shift of the C-O stretching frequency, +1 cm(-1). The results allow us to assign the feature observed at 2148 cm(-1) in the IR spectrum to CO at step sites. The other features of the spectrum at 2200, 2159 and 2152 cm(-1) are due to CO adsorbed at corner, edge, and terrace sites, respectively, as found in other studies. The results for the step suggest that for some surface defects the direct relationship between strength of the surface electric field and frequency shift is no longer valid because of inhomogeneities in the field.