Catalytic CO oxidation on the RuO2(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O-2 at 300 K, STM shows that unreactive features accumulate with each CO and O-2 titration run. HREELS and TDS show formation of increasing amounts of H2O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O-2 dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H2O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H2O desorption. (C) 2004 American Institute of Physics.