The fate and behavior of redox-active chemical species in geothermal systems is linked with the metabolic processes of chemotrophic thermophilic microorganisms. The major goal of the current work was to perform a thorough geochemical analysis of redox active species in geothermal outflow channels, and utilize these measurements to quantify the Gibbs free energy (ΔG_rxn) values for numerous oxidation-reduction reactions that represent potential chemolithotrophic metabolisms. Insights gained from energetic analyses can be used to structure hypotheses regarding novel microbial metabolisms and to guide cultivation strategies for isolating relevant microorganisms. A comprehensive suite of geochemical parameters, including major ions, trace elements, redox-active species and dissolved gases, were analyzed and monitored in vertical transects of 11 geothermal outflow channels in Yellowstone National Park from 2003-2005. The geothermal springs chosen for this study contained strikingly different aqueous and solid-phase geochemistry. These systems exhibited a wide range of conditions, including ranges in pH (2.7 to 7.0), temperature (60 °C to 92 °C), Cl^- (0.01 to 23 mM)), SO₄^2- (0.4 to 7.5 mM), NH₄⁺ (0.02 to 5.7 mM), CO₂ (aq) (0.1 to 4.5 mM), Fe (0.2 to 230 µM), and As (0.03 to 130 µM). The predominant changes in geochemistry occurring within geothermal outflow channels were consistently related to the dynamics of air-water gas exchange. In all springs studied, dissolved gases including H₂, CO₂, CH₄ and H₂S decreased down gradient of geothermal discharge, while O₂ ingassing resulted in increases in dissolved O₂. Calculated free energy (ΔG_rxn) values for balanced oxidation-reduction reactions suggest that numerous electron donor/acceptor combinations are exergonic in these systems. Reactions where H₂, CH₄, H₂S, S⁰, As^III, Fe^II and or NH₄ serve as electron donors were all significantly exergonic (< -30 kJ mole^-1 electron at all sites) when O₂ was the electron acceptor, even when O₂ levels were at the detection limit (3 µM). The geothermal systems included in this study all exhibited significant changes in microbial population distribution from near source-water conditions to sediments lining the outflow channels, consistent with the hypothesis that geochemical gradients and temperature correlate with microbial population distribution.