The oxidation of room temperature Si(111) by oxygen clusters (O-2)(n)(+) (n similar to 1600) at impact energies of 1.5, 3.0, and 4.5 keV has been examined by x-ray photoelectron spectroscopy. The impact of an oxygen cluster on clean Si(111) at these energies results in the oxidation of an area approximately equal to the cross sectional area of the cluster. Both suboxide and SiO2 are formed. The amount of oxide produced increases with the impact energy. Further exposure of the oxidized surface to the impact of oxygen clusters results primarily in an increase in the amount of SiO2. The number of silicon atoms oxidized per cluster impact on an oxidized surface is substantially less than for clean Si(111) and shows a strong dependence on the impact energy.