Amorphous non-oxide a-SiCxNy:H thin films have been synthesized at 300 or 500K from an Ar/HMDSN/NH3 gas mixture in a lab-scale microwaves PECVD reactor. These thin films have been characterized by FTIR spectroscopy and spectroscopic ellipsometry coupled with gas adsorption, in order to evidence the influence of PECVD synthesis conditions on the materials composition and microstructure. Deposition at high temperature (500K) was found to yield more inorganic and more compact films in comparison with those obtained at room temperature. In addition, when considering long term aging at room temperature for more than 1 month, the films synthesized at high temperature were found to be more stable toward oxidation in (humid) air.