We have modeled plasma-assisted diamond growth on substrates placed in a high velocity 1-dimensional flow. The gas consisted of methane or acetylene injected into a flow of partially dissociated hydrogen gas at 800 °C. Diamond is formed only near the injector. More diamond is formed when methane is the additive, and Raman spectra show that the quality of the diamond films is also higher when methane is the additive. The model, which includes detailed chemistry, convection, concentration diffusion, and thermal diffusion, shows that with this experimental arrangement only methane and methyl radicals are present in significant quantities when methane is added, while only acetylene is present when acetylene is added. We conclude that (1) Diamond films can be grown directly from methyl radicals (or, possibly, from methane) and from acetylene. This suggests that a variety of hydrocarbons could act as growth species. (2) An environment containing methane and methyl is much more effective for growing diamond films than one containing acetylene. (3) The quality of the diamond film depends on the identity of the growth species, with acetylene producing lower quality films than methyl (or methane). (4) The fall-off in diamond formation with distance from the injector is due to destruction of species crucial to diamond growth on the silicon substrates.