Electron-energy loss (EELS) spectra and thermal desorption (TDS) traces of carbon monoxide bound to the (100) surface of aluminum are presented. CO chemisorption on clean Al(100) is characterized by vibrational bands at 440 and 2060 cm-1 and by desorption at 125 K. Oxide "islands", formed by oxidation in O2 at 575 K, have no observed electronic influence on open metallic areas of the adsorbent but merely block CO adsorption sites.