The thermal stability of nanometric lanthanum aluminum oxynitride films on silicon was investigated in vacuum and oxygen. Isotopic enrichment of the films with N-15 and of the annealing atmosphere with O-18 combined with nuclear reaction analysis and Rutherford backscattering spectrometry provides direct evidence of atomic transport starting at 600 degrees C. Oxygen exchange and nitrogen replacement are identified. Interfacial silicon oxide growth takes place as the interfacial N concentration falls below 5 X 10(21) cm(-3). Complementary X-ray photoelectron spectroscopy shows changes in chemical bonding as a function of annealing temperature and ambient. (C) 2007 The Electrochemical Society.