We have performed new quantum calculations based on density functional theory, using plane waves and ultrasoft pseudopotentials to deal with the problem of modeling CO adsorption on a water ice surface and to obtain a theoretical understanding on the physical mechanisms involved during the monolayer formation. We have also done a new series of experiments, using the combination of volumetric adsorption isotherm and infrared spectroscopy measurements to study the different stages of CO adsorption on amorphous ice, from submonolayer to solid condensation. The multilayer formation has been evidenced by volumetric measurements, and its contribution has been characterized in the low-frequency part of the CO infrared spectrum. Three surface sites have been identified by spectroscopy, and their intensities have been controlled throughout the whole monolayer fort-nation, showing a site specific, though nonselective, adsorption process. Calculations have consistently succeeded in determining three adsorption configurations whose energies are similar. Finally, the geometry of the complete monolayer has been fully described, and its high compacity has been explained by analyzing the balance between vertical and lateral interactions.