Adsorption and oxidation of carbon monoxide have been studied on Ru(0001) single crystal and polycrystalline surfaces in sulfuric acid solutions by in situ Fourier transform infrared spectroscopy and electrochemical techniques. Carbon monoxide adsorbs on Ru( 0001) in linear and threefold bonding configurations, while only a linear configuration has been observed on polycrystalline surfaces. No CO oxidation can be discerned from the infrared spectra, because no CO2 band is observed up to 0.9 V, in sharp contrast with the facile reaction on polycrystalline surfaces. Voltammetry shows a negligible oxidation at very positive potentials. Oxidation of Ru( 0001) surface is limited to a one electron per Ru atom process, distinctly different from a high degree of oxidation of polycrystalline Ru. A limited oxidation of Ru( 0001) is likely the cause of the inactivity of Ru( 0001) for CO oxidation, which suggests an important role of the Ru surface structure and the form of ruthenium oxide in catalyzing CO oxidation.