Smooth and defect-rich Ru( 0001) surfaces prepared under ultrahigh-vacuum ( UHV) conditions have been loaded with oxygen under high-pressure ( p e 1 bar) and low-temperature ( T < 550 K) conditions. On these surfaces the CO oxidation reaction has been investigated by means of thermal desorption spectroscopy ( TDS), ultraviolet photoelectron spectroscopy ( UPS) and reactive molecular beam scattering ( RMBS). Both surfaces are oxide-free and exhibit a high reactivity. The maximum CO/CO2 conversion probability observed for a defect-rich Ru( 0001) surface amounts to 6 x 10(-3) and is comparable to that of a surface covered with rutile RuO2( 110) domains. RMBS experiments led to the identification of three different reaction channels. The first and second channel is related to CO adsorbing at oxygen-free defect sites and follow the Langmuir-Hinshelwood mechanism. Whereas the first reaction channel is already observed at room temperature, the second is thermally activated, contributing to the CO2 yield only for reaction temperatures above 400 K. The third channel is due to the recombination of CO molecules with oxygen atoms located in smooth areas of the surface undisturbed by defects. This reaction channel is thermally activated as well.