The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO 2) catalysts is that the reaction occurs on metal sites at the Au/TiO 2 interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO 2 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/TiO 2 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 TiO 2 sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.