The effects of nitrogen addition on the proper-ties of fluorinated amorphous carbon (a-C:H:F) thin films produced by radiofrequency plasma enhanced chemical vapor deposition have been investigated. The modifications of the structural and optical properties of the material upon thermal annealing were analyzed by: Raman spectroscopy; X-ray photoelectron spectroscopy: ellipsometry as well as measurements of the film thickness. It has been found that nitrogen addition can thermally stabilize, in terms of better dimensional stability, the as-deposited films. X-Ray photoelectron spectroscopy combined with Raman measurements revealed that for the films grown with nitrogen dilution, nitrogen is bonded to carbon within a graphitic framework and that a nitrogen loss occurs upon thermal treatment evolving in a more stable graphitic configuration. The variation of the refractive index after annealing has been correlated to structural changes due to the desorption of both fluorine and nitrogen from the film. The experimental findings suggest that carrying out a thermal treatment, the films grown with the highest nitrogen fraction undergo a reduction of the refractive index to a value that is comparable with the one obtained for the nitrogen-free a-C:H:F film.