We present an X-ray reflectivity study of thin TiN and TiOxNy films grown on Si(100) substrate by the dual-ion-beam-sputtering technique. The thickness range of the deposited films varies between 25 and 35 nm. We demonstrate that low-angle X-ray reflectivity is a powerful tool for studying the early stages of growth and the surface and interface properties of these films. We found that an oxidation of the TIN film surface occurs, yielding a 2 nm thick TiO2 film, if the films are exposed to air after the growth. Furthermore, an interface layer 1.5 nm thick with electron density near that of SiO2 is present at the substrate-him interface. The thickness of the interface layer increases by more than three times after 7-8 months (aging). The formation of the oxide layers is independent of the growth mode, i.e. growth with and without ion assistance. In addition, the formation of a TiO2 surface layer is also observed for TiOxNy films. The samples synthesized by employing ion-beam assistance show a much lower surface roughness (similar to 0.8 nm) compared to the samples deposited without ion-beam assistance (similar to 1.5 nm). This finding demonstrates that films with improved structural and surface properties can be obtained by using dual-ion-beam-sputtering.