Hydrofluorinated amorphous silicon-carbon alloys (a-Si1-xCx:H,F) are obtained by plasma decomposition of SiF4-CH4-H-2 mixtures. The analysis of the plasma phase, by mass spectrometry and optical emission spectroscopy, and of the resultant material, by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, allows us to gain information about the film growth chemistry. The growth kinetics and the material composition of the a-Si1-xCx:H,F films are studied as a function of the added CH4 amount to the SiF4. The peculiarity of SiF4-CH4 system is that small CH4 addition (10%) to SiF4 produces silicon carbon alloys with high C incorporation (60 at. %). This has been explained on the basis of a growth model in which the chemisorption of CHn on the surface prevails on that of SiFn.