RuO2-based catalysts are much more active in the oxidation of CO than related metallic Ru catalysts. This high catalytic activity (or low activation barrier) is attributed to the weak oxygen surface bonding of bridging 0 atoms on RuO2(110) in comparison with the strongly chemisorbed oxygen on Ru(0001). Since the RuO2(110) surface is able to stabilize an even more weakly bound on-top oxygen species, one would anticipate that the catalytic activity will increase further under oxidizing conditions. We will show that this view is far too simple to explain our temperature-programmed reaction experiments, employing isotope labeling of the potentially active surface oxygen species on RuO2(110). Rather, both surface O species on RuO2(110) reveal similar activities in oxidizing CO.