Thin films of yttrium-stabilised zirconia (YSZ) were prepared by ion-beam-assisted deposition (IBAD) onto Si(100) combining ion beam sputtering of an YSZ target with the bombardment of the growing films with argon ions or a mixture of argon/oxygen ions (energies 50-350 eV). In-situ Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) were employed to study the influence of IBAD on the growth process and the film stoichiometry, while the epitaxial quality was analysed rx situ by X-ray diffractometry (XRD). The oxygen content of the films is found to be strongly dependent on the ion energy, the ion-to-atom arrival ratio, as well as on the ion species. Whereas sputtered zirconia films were sub-stoichiometric, the oxygen content could be increased by bombarding the growing films with low-energy Ar+ ions. This increase in the oxygen content is lower if a mixture of argon/oxygen or even pure oxygen is used. Together with the structural results, one is led to a model of ion-induced film growth involving (i) preferential sputtering of oxygen at the film surface, resulting in (ii) enhancement of the mobility of the Zr cations and a higher order of the cation sublattice giving rise to increased oxygen diffusion into the film interior, followed by (iii) reoxidation at the surface.