||This study provides a link between thermodynamic calculations and observations of organic compounds in natural hydrothermal systems. Formate, acetate and succinate were quantified in hot spring fluids from Yellowstone National Park. Analytical concentrations of these molecules were used to evaluate their potential as energy sources for metabolisms of heterotrophic thermophilic microorganisms. The isotopic compositions of bulk dissolved organic and inorganic carbon were measured as a means to identify possible sources of carbon in hot spring fluids. These data are coupled to the isotopic composition of water samples collected simultaneously at the same springs to link carbon sources with the sources of the water that supplies the hydrothermal springs. Finally, a technique for estimating standard state thermodynamic properties of aqueous hydrocarbons was developed. The properties were then used in thermodynamic calculations to predict hydrocarbon solubilities, and to evaluate relative stabilities compared with naturally occurring redox buffering mineral assemblages.