This article describes medium-energy ion scattering (MEIS) measurements of [N] and [O] profiles using protons incident on nitride-oxide (NO) films produced by oxidizing Si(100) substrates in N2O plasmas and nitride-oxide-nitride stacks formed by depositing silicon nitride on the NO films. Concentrations were obtained from the MEIS spectra using a multiparameter Marquardt-Levenberg fitting procedure. Integral [N] and [O] were separately measured by nuclear reaction analyses and compared to the concentrations obtained from the MEIS profile. The analysis shows that nitrogen diffuses through oxide films during the deposition of silicon nitride by electron-cyclotron resonance chemical-vapor deposition and accumulates at the Si/SiO2 interface.