The steady-state rate r(CO2) of CO2 formation from CO + O-2 on an epitaxially grown RuO2(110) single-crystal surface was recorded as a function of the partial pressures in the 10(-7)-10(-6) mbar range. The RuO2(110) surface exposes singly coordinatively unsaturated "Ru-cus" sites and 2-fold coordinatively unsaturated "Ru-bridge" sites. Normally, the Ru-bridge sites are saturated by oxygen (O-bridge) which can be replaced by CO. The stage of the surface was controlled by vibrational spectroscopy (HREELS), and analysis of the kinetic data was based on previous information about the structural and adsorptive properties of this surface. Measurements of the rate as a function of temperature up to approximate to350 K in a 1:1 mixture of CO and O-2, each with 10(-7) mbar partial pressure, revealed most remarkable agreement with data reported by Zang and Kisch(13) with small supported RuO2 particles prepared from aqueous solution for 1 bar total pressure. Under these conditions of temperature and ratio of partial pressures the reaction will essentially proceed between CO and O species adsorbed at Ru-cus. The total pressure becomes largely insignificant: whereas the pressure changes by 10 orders of magnitude, r(CO2) is estimated to vary only within 1 order of magnitude se that the pressure gap is bridged here.