This paper reports the first observation, using in situ FTIR spectroscopy, of the oxidation of CO adsorbates on the Ru(0001) electrode to CO2 under open circuit (oc) conditions in both perchloric acid and sulphuric acid solution at 20 and 55 degreesC. While the significant oc oxidation of the adsorbed CO on the Ru(0001) electrode was observed in perchloric acid solution, much less oc oxidation took place in sulfuric acid solution due to the specific adsorption of bisulfate at the Ru surface which inhibits the surface oxidation and reduces the reactivity of the surface towards the oxidation of COads. The oc oxidation of the COads, depends strongly on the oxygen concentration in the solution and the temperature. The data so obtained are compared to those observed at the gas\solid interface, as well as to those obtained from the electro-oxidation of COads, and possible new catalytic oxidation reaction mechanisms are discussed. In addition, it is shown that the C-O frequency of the adsorbed CO may be used as an effective probe of the open circuit potential. (C) 2003 Elsevier B.V. All rights reserved.