In this work we investigated the structural and chemical properties of amorphous silicon oxynitride thin films, with distinct composition, deposited by plasma enhanced chemical vapor deposition from nitrous oxide and silane gas precursors. The utilized characterization techniques were Rutherford backscattering spectroscopy, X-ray absorption spectroscopy at the Si K-edge, and Fourier transform infrared spectroscopy. The results show a silicon first coordination shell composed of silicon, oxygen and nitrogen atoms, in a proportion that fits their atomic content. The Si-Si bonds are found only in samples having silicon content higher than ~50 at.%. These films also present higher amount of Si-H bonds. The film having the highest Si content, which shows evidences of Si clustering, was annealed in vacuum at 550, 750 and 1000 ℃. The results demonstrated that the annealed film is chemically stable under heat treatments in vacuum up to 1000 ℃. The hydrogen is totally released at 750 ℃. The main effect of the annealing process is to increase the segregation of silicon clusters.