Using a special detector setup in elastic recoil detection measurements, the incorporation of nitrogen during rapid thermal processing of Si(100) in N2O has been quantified for the first time. During oxidation at 1150 °C, the equivalent of a monolayer of silicon nitride is formed at the SiO2/Si interface. This retards the oxidation rate but it does not inhibit further oxide growth, which implies that gate oxides with thicknesses up to several tens of nm can be produced in N2O.