The mechanism of thin (<8 nm) oxide growth on Si(100) has been studied by high‐resolution medium energy ion scattering in combination with oxygen isotope substitution in theT=800–900 °C and 0.1–1 Torr oxygen pressure regime. Isotopic labeling experiments demonstrate that the Deal–Grove model breaks down for these films. In addition to the traditional oxidation reaction at the Si/SiO2interface, two other spatially specific reactions take place during thermal oxidation: an exchange reaction at the oxide surface and an oxidation reaction in the near‐interfacial region. ©1995 American Institute of Physics.