Semiempirical molecular orbital calculations are performed to study the physisorption of carbon monoxide molecules at the MgO (100) surface. This surface is simulated by clusters of different surface and layer sizes. Besides the submonolayer coverage four different types of monolayers are investigated. The stability and structure of the adsorbate system are determined, An adsorption energy per unit cell of the substrate is defined for a comparison of the stability of different monolayers with varying coverages. Agreement is found between the theoretical and experimental overlayer structure.