Silicon oxynitride films were deposited by plasma‐enhanced chemical‐vapor deposition. The elemental composition was varied between silicon nitride and silicon dioxide: SiO0.3N1.0, SiO0.7N1.6, SiO0.7N1.1, and SiO1.7N0.5. These films were annealed in air, at temperatures of 40–240 °C above the deposition temperature (260 °C), to determine the stability and behavior of each composition. The biaxial modulus, biaxial intrinsic stress, and elemental composition were measured at discrete intervals within the annealing cycle. Films deposited from primarily ammonia possessed considerable hydrogen (up to 38 at. %) and lost nitrogen and hydrogen at anneal temperatures (260–300 °C) only marginally higher than the deposition temperature. As the initial oxygen content increased a different mechanism controlled the behavior of the film: The temperature threshold for change rose to &bartil;350 °C and the loss of nitrogen was compensated by an equivalent rise in the oxygen content. The transformation from silicon oxynitride to silica was completed after 50 h at 400 °C. The initial biaxial modulus of all compositions was 21–30 GPa and the intrinsic stress was −30 to 85 MPa. Increasing the oxygen content raised the temperature threshold where cracking first occurred; the two film compositions with the highest initial oxygen content did not crack, even at the highest temperature (450 °C) investigated. At 450 °C the biaxial modulus increased to &bartil;100 GPa and the intrinsic stress was &bartil;200 MPa. These increases could be correlated with the observed change in the film’s composition. When nitrogen was replaced by oxygen, the induced stress remained lower than the biaxial strength of the material, but, when nitrogen and hydrogen were lost, stress‐relieving microcracking occurred. ©1995 American Institute of Physics.