Abstract Adsorbed states of CO on the Si(100)-K surface have been studied using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Three adsorbed states of CO (α, α 0 and β) were found on clean Si(100)-c(4 × 2) formed at 90 K. α(β)-CO is characterized by the CO stretch energy of 260 (−) meV and thermal desorption at 190 (415) K. The preadsorbed K atoms block the adsorption sites of α-CO and weaken the SiC bond of the coadsorbed α-CO. These results indicate that α-CO is predominantly bonded by the donation of 5σ electrons of CO into the π∗ surface state localized mainly at the down atom of a Si dimer. The preadsorbed K atoms block some adsorption sites of β-CO, but have little effect on the chemical bond between β-CO and Si. These results indicate that the adsorption sites of β-CO are the defects on Si(100). α 0-CO is characterized by the thermal desorption at 235 K, and is also associated with defects. Redshift is observed of the CO stretch energy of α-CO with increasing K coverage, which is mainly attributed to the vibrational Stark effect caused by the preadsorbed K adatoms.