Silicon oxynitride (SiON) films of similar to 1 mu m thickness were made by dip coating and subsequent thermal treatment of a preceramic polymer: perhydropolysilazane (PHPS). X-ray photoelectron spectroscopy was used to analyze the surface of a series of SiON films annealed in air between room temperature and 800 degrees C. Results reveal that the progressive changes in the bonding states of silicon, oxygen, and nitrogen due to different degrees of annealing lead to a systematic evolution of atomic ratios of the films. It is shown that PHPS forms a unique amorphous SiOxNy material upon annealing in air up to 800 degrees C with evolving compositions, instead of a mixture of Si3N4 and SiO2.