We have applied ambient-pressure x-ray photoelectron spectroscopy with Si 2p chemical shifts to study the real-time dry oxidation of Si(100), using pressures in the range of 0.01-1 Torr and temperatures of 300-530 °C, and examining the oxide thickness range from 0 to ∼25 A. The oxidation rate is initially very high (with rates of up to ∼225 Ah) and then, after a certain initial thickness of the oxide in the range of 6-22 A is formed, decreases to a slow state (with rates of ∼1.5-4.0 Ah). Neither the rapid nor the slow regime is explained by the standard Deal-Grove model for Si oxidation.