We have investigated by numerical simulations the gas-phase chemistry of a typical radio frequency, low pressure CH4/Ar plasma used for the deposition of diamond and diamond-like carbon films. We find that CH3 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 CH3 in methane plasmas, and C-2 in CH4/Ar plasmas.