The interaction of the K-covered Si(100) (2 X 1) surface with oxygen has been investigated by the use of high-resolution electron-energy-loss spectroscopy and thermal-desorption spectroscopy. It was found that the mechanism of enhanced oxidation of the K-modified Si(100) (2 X 1) surface is dependent on the fractional K coverage THETA(K). At low coverage THETA(K) less-than-or-equal-to 1). K atoms reduce the work function of the Si surface, thus increasing the initial sticking probability of oxygen; the oxygen atoms are bonded to Si and potassium oxides are not formed. However, at high coverage (THETA(K) > 1), K adatoms are bonded to oxygen and potassium oxides (K2O2, KO2) are formed; the promoting mechanism is related to the thermal decomposition of these oxides which play the role of an "oxygen reservoir."