The reaction of CO2 to carbonate CO3delta- is studied on the O-enriched RuO2(110) surface using thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy. It is known that the epitaxially grown RuO2(110) surface exposes coordinatively unsaturated sites, so-called Ru-cus and O-bridge, and can be O-enriched by dissociative adsorption of O-2 giving rise to weakly bound O-cus atoms on top of Ru-cus. CO2 adsorption at 85 K and annealing up to 250 K, results in a stepwise increased carbonate CO3delta-formation which takes place only on Ru-cus sites. Based on isotope substitution experiments the carbonate-related losses are identified among them the symmetric stretching mode at 150.8 meV and the asymmetric one at 174.9 meV. Through interaction of CO2delta- with O-cus, both chemisorbed on neighboring Ru-cus sites, a bidentate transient state and finally a monodentate carbonate CO3delta- is formed. The molecular plane of monodentate CO3delta- is oriented perpendicular to the surface with a tilted RuO-CO2 axis. The maximum carbonate coverage is about 25%.