Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x < 4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (T-p < 850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900-1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO2 coating layer manifests itself by LEED patterns characteristic for a particular RuO2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO2. (C) 2000 American Institute of Physics. [S0021-9606(00)71010-5].