The electronic structure of the Pt-modified Ge(0 0 1) surface with adsorbed CO molecules is studied by means of total-energy calculations within the density-functional theory. The potential energy surface of the structure is calculated and the preferable position for CO adsorption is determined. The CO molecule is found to adsorb atop the Pt wires, and to lead to the opening of a small band gap. The CO adsorption does not heavily affect the substrate morphology.