In order to elucidate the unusual catalytic behavior of ruthenium in the oxidation of CO, we have studied this reaction on a Ru(001) single crystal surface at high pressures with in-situ Fourier Transform-Infrared Reflection Absorption Spectroscopy (FT-IRAS). From the measurement of the steady-state surface coverages we are able to obtain estimates of mean residence times of the reactants. Under highly oxidizing conditions, where an O-(1 x 1) layer is present at the surface and the CO2 formation rate exhibits a maximum, we find CO residence times in the order of 10(-12) s. This suggests that the oxidation of CO may occur by direct reaction between gas-phase CO and chemisorbed oxygen atoms.