The carbon monoxide (CO) oxidation reaction, taking place over oxygen-rich Ru(0001) surfaces, has been used here as a simple probe of the reactivity of oxygen species formed by high temperature oxidation procedures. For surfaces oxidized at temperatures lower than 950K a high reaction probability, within a range of 10 - 2 , has been found. The kinetics of CO 2 emission exhibit a fast and a long term component which reflects the contributions of the topmost and the subsurface oxygen phases, respectively. Both kinetic components are drastically reduced when working with surfaces resulting from the application of the same oxidation procedure but at temperatures above 950K. For preparation temperatures above 1100K the reaction probability is below the detection limit. In this temperature range, the oxygen phases show valence region ultraviolet photoelectron spectroscopy features characteristic of a regular RuO 2 solid phase. Moreover, these non-reactive surfaces are marked by intense emission of RuO 3 , RuO 4 and Ru 2 O molecular fragments as appearing in thermal desorption spectra in a temperature range 1000-1300K.