We have studied optical and structural properties of GeOx films produced by reactive DC magnetron sputtering of Ge targets in an Ar/O-2-mixture. Optical spectroscopy measurements of reflectance and transmittance of the films were employed to determine optical properties. The film structure was determined by X-ray diffraction measurements, while X-ray reflectometry was used to determine the thickness, density, and roughness of the films. The him topography was additionally characterized by atomic force microscopy. For appropriate oxygen flows highly transmitting GeOx films can be grown at rates up to 3 nm/s for power density of 2.6 W/cm(2). Optical and structural properties of the films are closely correlated and can be controlled by the oxygen dow. With increasing oxygen how an increasing growth rate of the oxide films is initially observed. The resulting films above 20 seem and below 25 seem O-2 are transparent in the visible range, have a negligible roughness and are amorphous. A further rise in oxygen how lends to a pronounced decrease in growth rate above 25 seem and a further increase in the bandgap of the germanium oxide up to mon than 48000 cm (1) (5.95 eV). This transition in him growth is accompanied by an increase in roughness and a structural transition leading to polycrystalline films of alpha-GeO2 (quartz-structure). Nevertheless, the film density is hardly affected by the structural change and remains rather constant around 3.65 g/cm(3), which is 85% of the density of single crystalline alpha-GeO2. A model is presented which can account for the change of film properties.