We have investigated by numerical simulations the gas-phase chemistry of a typical radio frequency, low pressure CH 4 /Ar plasma used for the deposition of diamond and diamond-like carbon films. We find that CH 3 is the most abundant carbon containing radical in pure methane discharges, while it is the carbon dimer C 2 in discharges of methane highly diluted by argon. Thus, we propose that the gaseous precursor of the film is CH 3 in methane plasmas, and C 2 in CH 4 /Ar plasmas.