We summarize some recent work on the formation mechanisms, structure and composition of oxynitride and high-g films, as investigated by high-resolution medium-energy ion scattering (MEIS). We show that nitridation of a silicon oxide thin film takes place through transport of NO molecules to the oxide/silicon interface. Ta2O5 films on Si have a compositionally graded oxide, breaking up at high annealing temperatures. A thin buffer layer of Si3N4 can prevent this.