Surface infrared spectroscopy is used to examine the initial phases of oxidation of hydrogen-terminated Si(100) in ultrapure water containing dissolved oxygen. Analysis of both Si−O and Si−H vibrational modes suggests that oxide growth occurs in patches and reveals that much of the surface remains unoxidized after 5 h of immersion in O2(aq). Isotopic labeling experiments are used to demonstrate that the same type of surface etching that takes place in O2-free water occurs in the presence of O2(aq). Evidence for surface homogenization during the earliest stages of this etching process suggests that etching might play a critical role in smooth oxide growth, a conclusion supported by the near absence of detectable surface oxidation when etching is virtually eliminated by lowering the solution pH.