Abstract Thermodynamic analysis has been made in order to evaluate the effect of the chemical vapor deposition (CVD) processing variables on the driving force for precipitation of carbon from the gas phase in the C-H, C-O and C-H-O systems. It is suggested that when the chemical reactions are involved, the chemical potential of carbon in the gas phase increases monotonically as the chemical reactions proceed and becomes maximum in the metastable gas phase equilibrium. The maximum supersaturation ratio for deposition of diamond is suggested to be the ratio of the partial pressure of carbon in the metastable gas phase equilibrium to the equilibrium vapor pressure of diamond. The lines of the iso-supersaturation ratio are added to the CVD phase diagram. The phase boundary of the CVD phase diagram corresponds to the line of the iso-supersaturation ratio of unity.