The theory presented explains quantitatively the experimentally observed increase in film density of a vapor-deposited CeO2 film when bombarded during growth with low-energy O 2 + ions. The density enhancement is expressed in terms of the yields for recoil implantation of surface atoms, ion incorporation and sputtering, which have been determined by employing a three-dimensional Monte Carlo cascade calculation. Ion-to-vapor flux ratios between 0 and 1.4 and O+ ion energies between 25 and 600 eV have been examined. The density shows an almost linear increase with the ratio of ion-to-vapor fluxes. An optimum O+ ion energy for densification is found at about 200 eV which is in agreement with experiment.