We annealed ultrathin silicon oxynitride films at high temperatures and in high vacuum, and investigated a role of nitrogen in structural stability with angle resolved X-ray photoelectron spectroscopy. The desorption rate of oxygen from the oxynitride films is much less than that from oxide films at 800 degreesC, Incorporated nitrogen does not desorb from the films, although Si-N bonds are gradually broken above 800 degreesC. Incorporated nitrogen enhances the thermal stability of the oxynitride films.