Secondary ion mass spectrometry was used to profile deuterium concentrations [D] in 100‐200‐nm‐thick SiO2films, which were grown by thermal oxidation of silicon in D2O steam pressures of 0.05‐8 atm and at temperatures of 550‐1000 °C. Deuterium was used to simulate hydrogen chemistry in steam oxidations and is readily detected in the presence of high background concentrations of hydrogen in the sputtering ambient. Structure observed here in D profiles, and previously in H profiles, near the SiO2‐Si interface are due predominantly to abrupt changes in ion yields as the surface charging conditions change. The 1‐atm profiles generally exhibit an increase in [D] in the SiO2toward the Si interface; the average [D] is nearly constant at 0.6‐1.0×1020cm−3for oxidations at 800‐1000 °C and rises to 6×1020cm−3at 600 °C. It is argued that the D is predominantly in the form of Si‐OD. At 800 °C and 0.05‐8 atm, the average OD concentration in the oxide film is approximately proportional to (steam pressure)1/2. This is the first demonstration of the simple equilibrium reaction between ambient water and the as‐grown oxide layer.