A silicon wafer has been oxidized at room temperature in vacuum using a pure, ground‐state beam of O−ions. The beam was of sufficiently low energy that no displacement damage or implantation was energetically possible. The resulting SiO2films were analyzed with x‐ray photoelectron spectroscopy. A logarithmic dependence of oxide thickness on dose was observed, with an extrapolated oxidation efficiency of unity for the clean silicon surface. A distinct initial oxidation phase was observed, with an anomalously high level of silicon suboxides. In addition, the valence‐band offset between the silicon and the oxide was unusually small, suggesting a large interfacial dipole.