X-ray photoelectron spectroscopy (XPS) coupled with Fourier transform infrared (FTIR) and optical transmission spectroscopy (OTS) has been used for the characterization of silicon-carbon alloys (a-Si_1−xC_x: H, F) deposited via plasma, by varying the CH₄ amount in SiF₄–CH₄–H₂ feeding mixture. XPS measurements have shown that carbon-rich a-Si_1−xC_x: H, F alloys include large amounts of fluorine (>11 at. %), which make the films susceptible to the air oxidation. In addition, the effect of the alloying partner carbon on the valence band (VB) and on the VB edge position of amorphous silicon is also described.