We have investigated the effect of iron and oxygen fugacity on dolomite + coesite stability during subduction. For redox conditions buffered by the assemblage itself, the presence of iron (Fe/(Fe + Mg) ca. 0.4) lowers the decarbonation reaction by about 200 °C at 4.25 GPa and by 300 °C at 5.5 GPa, compared to the iron-free reaction. Clinopyroxene and CO2 form by a decarbonation reaction similar to the iron-free system. At low temperature, however, graphite replaces CO2 through redox interactions with iron in the carbonate. Melting occurs approximately 100 °C above decarbonation and a carbonatitic melt is produced. In a second series of experiments, we imposed lower oxygen fugacity by adding molybdenum, in order to study the potential redox mechanisms in contact with the peridotitic mantle. In these samples, we observe systematic carbon reduction producing the assemblage clinopyroxene + graphite. Our results show that the stability of dolomite + coesite on a subduction path is limited by redox interactions, in addition to pressure and temperature. As our experiments were run in the stability field of diamond, we also demonstrate that diamond may form from dolomite + coesite during subduction. Chemical diffusion of iron and oxygen in the slab and at the slab/mantle interface appears to be a key parameter to determine at what pressure–temperature conditions this may happen.