The adsorption of CO on RuO2(110) single-crystalline thin films grown on Ru(0001) was studied with scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and high-resolution electron energy-loss spectroscopy. The stoichiometric RuO2(110) surface exhibits alternating rows of O-bridge and coordinatively unsaturated Ru atoms (Ru-cus). We identify two different CO adsorption states: CO-bridge and CO-cus. CO-bridge is known to adsorb at the O-bridge positions after reacting-off O-bridge. CO-cus is linearly bonded to Ru-cus in an on-top position and becomes stabilized only after complete filling of the CO-bridge sites. The CO-cus state exhibits two desorption maxima, at 200 and 320 K, corresponding to a surface coverage of approximately 1.0 (alpha state) and 0.5 (beta state), respectively. At 300 K and without CO gas in the background, CO-cus remains only temporarily present at the surface, and reaches coverages of 0.5. A coverage of 1.0 can be achieved by exposure to CO at 300 K to prepare CO-bridge and further exposure to CO at 85 K. From time-dependent STM and from TDS the binding energy of the beta-state CO-cus is determined at about -1.0 eV. (C) 2003 American Institute of Physics.