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We report a joint experimental and theoretical study of CO chemisorption on the golden cages. We find that the Au17 − cage is highly robust and retains its cage structure in Au17 − (CO)−. On the other hand, the Au16 − cage is transformed to a structure similar to Au17 − upon the adsorption of CO. Au18 − is known to consist of two nearly degenerate structures, i.e., a cage and a pyramidal isomer, which coexist in the cluster beam. However, upon CO chemisorption only the cage isomer is observed while the pyramidal isomer no longer exists due to its less favorable interaction with CO, compared to the cage isomer. We find that inclusion of the spin-orbit effects is critical in yielding simulated spectra in quantitative agreement with the experimental data and providing unequivocal structural information and molecular insights into the chemical interactions between CO and the golden cages.