THERMAL maturation of organic matter in sedimentary basins results in the generation of numerous organic alteration products. These products influence the chemical and physical alteration processes of sediments(1-3), and in some instances their accumulation results in the formation of oil and natural-gas deposits(4,5). Much uncertainty exists about the factors that control the relative abundances of individual organic species during maturation. Although it is clear that kinetic barriers allow thermodynamically unstable species to persist in a metastable state for geologically significant periods of time, local equilibrium between more-reactive species may substantially influence their abundance. Here I report the results of redox-buffered hydrothermal experiments designed to investigate reactions that may occur in geological environments between ethane, ethene, water and inorganic redox-sensitive minerals. I find that reversible metastable thermodynamic equilibrium is attained between these species. Because water participates directly in this equilibrium, it may represent a reactive and abundant source of hydrogen for hydrocarbon generation in sedimentary basins. This demonstration that metastable equilibrium is attained implies that some organic geochemical reactions can be thermodynamically modelled and predicted despite the absence of total thermodynamic equilibrium.