When 3 L CO was dosed on Si substrates at 100 K, the CO molecule readily adsorbed on Si(100)-2 × 1, but caused no obvious change in the HREELS of Si(111)-7 × 7. The interaction between CO and Si(100)-2 × 1, however, is rather weak as indicated by the low Si-C stretching frequency (51 meV) and the small red-shift (8 meV) of the C-O stretching mode from its free molecular value. Nearly all of the CO molecules desorbed from the Si(100) when the surface was warmed to Ts = 220 K or when the surface was irradiated with 308 nm excimer laser for two 2 min. In the TPD spectrum, a desorption peak at 180 K was observed for m e = 29 (13CO). We report, for the first time, the vibrational spectrum of the CO molecule adsorbed on a Si single-crystal surface.