CO chemisorption on the metallic molybdenum nanoparticles supported on the thin alumina film was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A binary compound of molybdenum and CO is found to be formed on the surface upon CO dose, accompanied with a positive binding energy shift of the Mo 3d doublet and a localized Mo 4d valence band. A loose packing of the metallic molybdenum favors the formation of this intermediate Mo-x(CO)(y) species. The formation of the Mo-x(CO)(y) species implies that the property of the metallic molybdenum nanoparticles on the thin alumina film is much different from that of the bulk molybdenum, indicating a significant nanometer size effect.