Equilibrium calculations and published phase equilibria are used to show that at temperatures below 400 °C and pressures above 300 bar, the fluids coexisting with graphite in the C-O-H system consist either of CO2-H2O mixtures or CH4-H2O mixtures. Bulk fluid compositions consisting of CH4-H2O will unmix to methane-rich fluid and H2O-rich liquid at temperatures below about 325 °C. Compositions on the CO2-H2O join will unmix only at temperatures below about 275 °C. Oxygen fugacity in the CH4-H2O-graphite three-phase region is fixed (at constant P and T) and approximately equal to that of the quartz-magnetite-fayalite assemblage. In the CO2-H2O-graphite three-phase region, oxygen fugacity is about three orders of magnitude greater. From the end stages of diagenesis to temperatures of 300 °C (and possibly 400 °C in salt-rich systems), many metamorphic rocks may contain CH4-H2O fluids rather than CO2-H2O mixtures. Thus, metamorphic reactions involving carbonate minerals would involve CH4, graphite, and H2O rather than CO2. The immiscibility between CH4 and H2O could result in the common occurrence of methane (natural gas) in low-grade metamorphic terrains.