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The combination of noble metals and reducible metal oxides often displays superior catalytic properties in lowtemperature CO oxidation reactions. In the present study, we investigated the adsorption and activation of CO on submonolayer and monolayer CoO x films deposited on Ir(100) at low temperature under ultra-high vacuum (UHV) conditions. Using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we found that the CoO x films have a (3 × 1) structure and consist of a Co−O bilayer and an O−Co−O trilayer associated with Co 2+ and Co 3+ states, respectively. A combination of XPS, temperature-programmed desorption (TPD), and infrared reflection absorption spectrum (IRAS) reveals that the presence of the interface of the submonolayer CoO x film and Ir significantly enhances the catalytic activity of cobalt oxide for CO oxidation. In contrast, a monolayer film of CoO x without exposing CoO x −Ir interface sites is hardly active for CO oxidation. CO adsorbs on the bridge sites of Ir atoms on the submonolayer CoO x /Ir(100) surface at 100 K. Upon heating, CO partially reacts with oxygen on the surface to produce CO 2 , which desorbs at 215 K, while unreacted CO molecules adsorbed on Ir bridge sites migrate to top sites. Increasing the temperature leads to CO 2 desorption at 295 and 325 K, arising from the reaction of CO with chemisorbed oxygen on the Ir surface and oxygen of cobalt oxide, respectively. These results reveal the importance of the interface between Ir and cobalt oxide in the oxidation of CO.