It has been demonstrated that the oxidation of a Ru(0001) surface proceeds via four well characterized stages:(a) chemisorption, (b) oxygen penetration, (c) interfacial growth, (d) lateral and scale growth. The physical properties of the resulting oxygen phases were determined by means of thermal desorption spectroscopy (TDS), ultraviolet photoionization spectroscopy (UPS(21.2eV)) and low energy electron diffraction (LEED). The CO-oxidation reaction as performed over oxygen-rich surfaces has been used as a sensitive probe of various oxygen states. The analysis of the integral yield and the reaction kinetics reveals four well distinguishable oxygen species which are characteristic for the consecutive oxidation stages. In the high temperature regime (T>550K) the very efficient reaction is limited only by the diffusion of oxygen atoms from the subsurface region toward the topmost layer. The activation energy for this limiting reaction step ranges from 0.1 to 0.3 eV. In the low temperature regime (T<350K) only oxygen atoms weakly bound with small oxide domains participate in the reaction. This unique oxygen phase raises the integral CO/CO@sub 2@ conversion probability to a level of 10@super -1@. In contrast, surfaces containing a regular RuO@sub 2@ layers exhibit only a negligibly low reaction yield.