Metallic films of RuO2 were deposited by reactive-sputtering deposition on single crystal substrates of Al2O3 (0001) and SrTiO3 (100) at room temperature and 450 degrees C. Measurement of the characteristic hysteresis loop revealed that the target’s transition from a metal to a metal oxide surface occurred at a very high O-2/Ar ratio (88%) under our experimental conditions. The hysteretical behavior of the transition was evaluated experimentally and was modeled. Resonance-enhanced Rutherford backscattering spectrometry established that the films deposited at 450 degrees C had an oxygen to ruthenium ratio of 1.97, while a slightly higher value of 2.08 was observed for the films grown at room temperature. The latter films were amorphous, whereas those grown at 450 degrees C exhibited a highly oriented polycrystalline microstructure. On SrTiO3 (100), the RuO2 (100) plane is parallel to the substrate surface, but no in-plane orientation was found. The same face, RuO2 (100), was also parallel to the surface of Al2O3 (0001), and the [001] direction of individual grains of RuO2 aligned with the three (<(1)over bar 010>) directions of the substrate to produce a threefold mosaic microstructure.