Hydrothermal circulation can lead to fluid mixing on any planet with liquid water and a source of heat. Aqueous fluids with differing compositions, especially different oxidation states, are likely to be far from thermodynamic equilibrium when they mix, and provide a source of free energy that can drive organic synthesis from CO2 and H2, and/or supply a source of geochemical energy to chemolithoautotrophic organisms. Results are presented that quantify the potential for organic synthesis during unbuffered fluid mixing in present submarine hydrothermal systems, as well as hypothetical systems that may have existed on the early Earth and Mars. Dissolved hydrogen, present in submarine hydrothermal fluids owing to the high-temperature reduction of H2O as seawater reacts with oceanic crustal rocks, provides the reduction potential and the thermodynamic drive for organic synthesis from CO2 (or bicarbonate) as hydrothermal fluids mix with seawater. The potential for organic synthesis is a strong function of the H2 content of the hydrothermal fluid, which is, in turn, a function of the prevailing oxidation state controlled by the composition of the rock that hosts the hydrothermal system. Hydrothermal fluids with initial oxidation states at or below those set by the fayalite-magnetite-quartz mineral assemblage show the greatest potential for driving organic synthesis. These calculations show that it is thermodynamically possible for 100% of the carbon in the mixed fluid to be reduced to a mixture of carboxylic acids, alcohols, and ketones in the range 250--50 ¿C as cold seawater mixes with the hydrothermal fluid. As the temperature drops, larger organic molecules are favored, which implies that fluid mixing could drive the geochemical equivalent of a metabolic system. This enormous reduction potential probably drives a large portion of the primary productivity around present seafloor hydrothermal vents and would have been present in hydrothermal systems on the early Earth or Mars. The single largest control on the potential for organic synthesis is the composition of the rock that hosts the hydrothermal system. ¿ 1998 American Geophysical Union