Fluids rich in water and carbon (COH) are among the most reactive agents of mantle metasomatism and play a major role in the continuous degassing of the Earth. The addition of small amounts of H₂O and CO₂ depress the solidus of mantle peridotite by several hundred degrees, resulting in the common occurrence of highly mobile, volatile-rich melts, which are a main cause of heterogeneities in the Earth’s mantle. Most experimental studies have investigated relatively oxidizing conditions in which COH fluids are mixtures of H₂O + CO₂, whereas experiments on the partial melting of mafic and ultramafic rocks under reducing conditions, in which CH₄ + H₂O mixtures occur, remain sparse. We have performed experiments on eclogite and lherzolite compositions at pressures from 1.8 to 6 GPa, and temperatures ranging from 800 to 1600 ℃ at different oxygen fugacity conditions (FMQ - 5.6 to FMQ - 0.3), either with 5 wt% CH₄ + H₂O or 5 wt% H₂O added. Our results show that under reducing conditions (FMQ - 5.6 or IW - 0.7), the solidi of both eclogite and lherzolite with COH fluids are about 200 ℃ lower than the anhydrous solidi, but 100 ℃ higher than those under oxidizing conditions with 5 wt% H₂O. The mineral chemistry of residual solid phases in both systems depends on oxygen fugacity (fO₂). The stability of clinopyroxene at the solidus of lherzolite under reducing conditions is significantly greater than its stability under oxidizing conditions, whereas garnet is only stable during the partial melting of eclogite at low fO₂. The composition of partial melts of lherzolite with 5 wt% CH₄ + H₂O under reducing conditions at low pressures is quartz normative rather than olivine normative. The higher-temperature solidi and phase relations under reducing conditions indicate that, if Archean mantle was more reduced than the modern mantle, komatiite could be formed under reducing conditions with no need for deep melting or the presence of abundant water. Further, our experiments indicate that low fO₂ favors the precipitation of graphite or diamond, so a large amount of diamond or graphite could have accumulated in the Earth’s deep mantle and the Martian mantle.