Experimental data on phase transformations and melting in peridotite and eclogite systems with a C–O–H fluid at 6–30 GPa have been analyzed with special attention to the influence of redox conditions. It has been found that melting in systems with H2O depends heavily on its total content and considerably on its solubility in nominally anhydrous rock-forming minerals. Partial melting occurs when the total H2O content of the system exceeds the H2O storage capacity in the rock under given physicochemical conditions. Melting in CO2-containing systems is determined by carbonate stability and the chemical composition of the system, mainly its Na2O and K2O contents, and, to a smaller extent, the content of CO2 itself. Studies of peridotite and eclogite systems containing H2O, CO2, H2O + CO2, and a reduced C–O–H fluid show that most solidi flatten out at pressures above 6–8 GPa when intersecting the geotherms of subduction and average mantle. Mantle melting at constant pressure in the presence of a C–O–H fluid depends not only on temperature but also on redox conditions. The oxidation of the system causes redox melting. The stability boundary of a Fe–Ni alloy (it may coincide with the lithosphere–asthenosphere boundary under cratons, 200–250 km) and the 410-km discontinuity are paramount to redox and decarbonation–dehydration melting. Also, the paper provides evidence that subducted carbonates play the leading role in the “big” mantle wedge model for stagnant slabs. Volatile-containing eclogite systems melt at lower temperatures than peridotite ones (the difference is up to 100–200 °C). This suggests that eclogites are of global importance in mantle melting, which agrees with modern geochemical models.