The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO2) catalysts is that the reaction occurs on metal sites at the Au/TiO2 interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO2 during CO oxidation. Infrared-kinetic measurements indicate that O-O bond scission is activated by the formation of a CO-O-2 complex at dual Ti-Au sites at the Au/TiO2 interface. Density functional theory calculations, which provide the activation barriers for the formation and bond scission of the CO-O-2 complex, confirm this model as well as the measured apparent activation energy of 0.16 electron volt. The observation of sequential delivery and reaction of CO first from TiO2 sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.