The phase separation of Si:ZrO₂ and Ge:ZrO₂ films in superlattice geometries were investigated by X-ray diffraction and reflectometry as well as by high resolution transmission electron microscopy. In case of the Si containing films, round-shaped clusters within a crystalline ZrO₂ matrix have been observed after annealing at 1,000°C. The appearance of nanocrystalline Si could not be shown, whereas amorphous clusters within the crystalline ZrO₂ matrix were formed. For the Ge containing films, the formation of nanocrystalline Ge layers was observed after annealing at 650°C. In both material systems the ZrO₂ matrix crystallized in the tetragonal phase.