Silica supported Pt–Co and Au–Co nanoparticles (NPs) were subjected to various redox processes and characterized by X-ray diffraction, X-ray absorption near edge structure, and X-ray photoelectron spectroscopy. We found that most of the Co oxide (CoOx) species on Pt NPs can be reduced at 100 °C forming an alloy structure with Pt at elevated temperatures. Oxidation of Co in the reduced sample takes place gradually with increasing temperatures. In contrast, temperatures higher than 400 °C are needed to reduce CoOx on Au NPs and Co atoms hardly form an alloy with Au even at 600 °C. The Co species in the reduced Au–Co/SiO2 sample were quickly oxidized in an O2 atmosphere at room temperature. High CO oxidation activity was observed in the Pt–Co/SiO2 catalyst reduced below 300 °C; however this necessitated reduction at 600 °C of the Au–Co/SiO2 catalyst. The results illustrate a stronger interaction of Co (CoOx) with Pt than with Au. In both systems, the optimum treatment conditions are to produce a similar CoO-on-noble metal (NM) active structure and maximize the density of interface sites between the surface CoO structure and the NM support.