Here we present a combined operando infrared spectroscopic and theoretical analysis of ceria-supported gold catalysts during room temperature CO oxidation that identifies an active site for the reaction as a single gold site on the ceria support forming an O-lattice-Au+-CO species. As monitored by operando infrared spectroscopy, the isolated Au+ gold site is either present as a result of the catalyst synthesis or formed under reaction conditions after CO adsorption at the perimeter of the Au-ceria interface. Our results provide new insights into the chemical nature of the active gold site and the reaction mechanism by detecting the formation of active and inhibiting species simultaneously. (C) 2020 Elsevier Inc. All rights reserved.