Diamond formation was studied experimentally at 7.5 GPa and 1450-1600°C in the H2O-C, ÑÍ4-Í2-Ñ, SiO2–H2O–Ñ and Mg2SiO4–H2O–Ñ systems. Most favorable conditions for diamond formation occur in the water-rich fluid phase, containing small amounts of silicate solute. The degree of graphite to diamond transformation decreases with the decrease of water concentration in the studied systems. In the silicate systems at low H2O contents, spontaneous nucleation and growth of diamond is very weak. The change of the kinetics of diamond formation and the rate of carbon mass transfer is due to the decrease of molecular water concentration in the crystallization media. An increase in the contents of CH4 in fluid inhibits the nucleation of diamond and decreases the rate of its growth.