A comparison of several simple hydrocarbon gases, with H:C ratios ranging from 1 to 4, as precursors for the carbonization of Si is presented. The growth experiments were performed by RTCVD at reactor pressures of 760 and 5 Torr. For AP-RTCVD, we have found that C3H8, C2H4 and C2H2 have very similar dependence of growth rate on hydrocarbon partial pressure in the chamber. At 1300°C, this involved a maximum in film thickness being obtained at a hydrocarbon flow rate of 8–10 seem, representing a transition hydrocarbon fraction (in H2) of ∼ 5×10-4. CH4 carbonization produces a peak growth rate at a significantly higher fraction, ∼ 4×10-3. For LP-CVD at 5 Torr, the transition carbonization fraction increases by approximately an order of magnitude. The AP-RTCVD carbonization activation energy for C3H8, C2H4 and C2H2 at higher temperatures (∼1200–1300°C) has a common value of ∼ 0.8 eV, while for lower temperatures it depends on the hydrocarbon.