Mononuclear gold complexes synthesized from Au-III(CH3)(2)(acac) in zeolite NaY were characterized by time-resolved X-ray absorption spectroscopy and infrared spectroscopy as they catalyzed CO oxidation at 298 K and 760 Torr in flow systems. Initial contact with a CO + O-2 mixture led to the rapid formation of cationic gold complexes in which Au was bonded to approximately two zeolite O atoms, on average. Further contact with CO + O-2 led to breaking of an Au-surface oxygen bond, giving a gold carbonyl anchored to approximately one O atom. The process was reversed in the absence of CO and O-2.