The application of a-Si 1- xC x: H films, deposited by plasma techniques, in electronic devices operating at high current density and high temperature is limited by the metastability of the alloy. So it is of great interest to study the thermal stability of such films grown by plasma enhanced chemical vapor deposition (PECVD). Films of a-Si 1- xC x: H have been deposited by ultra-high vacuum PECVD in SiH 4 + CH 4 and SiH 4 + C 2H 2 gas mixtures. Optical, infrared, electron spin resonance and photoluminescence measurements have been performed on as deposited films and after annealing in the range 250-500°C. All the films, deposited by both CH 4 and C 2H 2 sources, have a strong optical, structural and defect density stability up to annealing temperature of 400°C. At higher temperatures only near stoichiometric samples are stable for what concerns optical and spin density properties. The correlation between experimental results and physical processes occurring during annealing is discussed.