The effect of coincident ion bombardment on the oxidation of Si (100) by atomic oxygen has been examined under ultrahigh vacuum conditions by employing molecular beam techniques and x‐ray photoelectron spectroscopy. Ion bombardment leads to a significant enhancement in the rate of oxidation. By modulating both the oxygen and ion (Ar+) fluxes several possible mechanisms for the enhanced rate can be eliminated. Of the remaining possibilities, a mechanism involving competition between ion‐induced oxygen incorporation and sputtering appears most likely.