The stability and reactivity of cationic gold-cerium oxide clusters, Au(m)ce(n)O(2n+x)(+) (m <= 4, n <= 7, -1 <= x <= 2), were examined experimentally and computationally. These clusters were generated by simultaneous laser ablation of gold and cerium oxide targets and analyzed by time-of-flight mass spectrometry combined with gas-phase temperature programmed desorption. Stable compositions of gold cerium oxide clusters were identified as AumCenO2n+ and AumCenO2n+1+ for m >= 1, containing one oxygen atom more than the stable gold-free cerium oxide clusters CenO2n-1+ and CenO2n+. In either case, the stable clusters mainly consisted of Ce4+ and O2-, and the gold atoms had an oxidation state of +1. The reactivity of cerium oxide clusters toward CO was modified by gold atoms, which hindered CO oxidation while efficiently promoting its adsorption. According to density functional theory calculations, the oxygen-centered radical of cerium oxide clusters, considered to be the reactive site, was geometrically and electronically inactivated by gold atoms, which functioned as a CO adsorption site.