With surface X-ray diffraction (SXRD) using a high-pressure reaction chamber we investigated in-situ the oxidation of the Ru(0001) model catalyst under various reaction conditions, starting from a strongly oxidizing environment to reaction conditions typical for CO oxidation. With a mixture Of O-2 and CO (stoichiometry, 2:1) the partial pressure of oxygen has to be increased to 20 mbar to form the catalytically active RuO2(110) oxide film, while in pure oxygen environment a pressure of 10(-5) mbar is already sufficient to oxidize the Ru(0001) surface. For preparation temperatures in the range of 550-630 K a self-limiting RUO2(110) film is produced with a thickness of 1.6 nm. The RuO2(110) film grows self-acceleratedly after an induction period. The RuO2 films on Ru(0001) can readily be reduced by H-2 and CO exposures at 415 K, without an induction period.