Equilibrium concentrations of various hydrocarbon species which existed in the vapor for growing solid carbon is calculated and a phase diagram for the carbon-hydrogen system is obtained using the thermodynamic quasi-equilibrium model. These calculations show that methane is the sole reaction product at temperatures below 1000℃, acethylene is, however, the dominant hydrocarbon speies above 2000℃. The CVD phase diagram shows that high methane concentrations and low total pressures decrease the lower temperature limit required to obtain any deposit. The fraction of carbon deposited from methane-hydrogen mixtures changes rapidly from practically zero at lower temperature limit to close to 100% over a couple of hundred degrees, then to decrease to zero at higher temperature limit.