The formation of CO2, by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated as a function of the oxygen coverage for sample temperatures up to 900 K. It turned out that the reaction probability per incident CO molecule is below 5 x 10(-4) for O coverages up to 3 monolayers (ML); oxygen in excess of 1 ML is located in the subsurface region. The reaction probability for the (1 x 1)-10 phase is in agreement with the data derived from high-pressure experiments by Peden and Goodman [J. Phys. Chem. 1986, 90, 1360]. Even for CO molecules with a translational energy of 1.2 eV (supersonic molecular beam experiments), the reaction probability is less than 5 x10(-2). This value is consistent with the activation barrier derived from DFT calculations for a reaction by direct collision from the gas phase (Eley-Rideal mechanism). Beyond an oxygen load of 3 ML, however, the reaction probability increases by 2 orders of magnitude. It is suggested that this enhancement is due to a further destabilization of the surface oxygen by the onset of oxide formation.