Acid-sulfate-chloride (ASC) springs (pH~3, 65-85°C) in Norris Geyser Basin, Yellowstone National Park, contain a suite of reduced chemical species that may serve as energy sources for thermophilic chemotrophic microorganisms. The predominant dissolved ions of ASC springs include Na⁺ (13-16 mM), Cl^- (13-16 mM), SO₄^2- (~1.4 mM), and H⁺ (~1 mM), as well as significant concentrations of Fe^II, AsI^II, H₂S(aq), CH₄(aq), CO2(aq) and H₂(aq). Aqueous chemistry, solid phase geochemistry, and temperature co-vary in the outflow channels of ASC springs and together act to establish niche opportunities for suitably adapted microorganisms. The primary objectives of our work on ASC springs are to describe and discover the adapted microorganisms that inhabit these springs, determine their metabolic strategies, and identify linkages between microbial population distribution and in situ geochemical processes. Our approach has combined thorough geochemical analysis of aqueous and solid phases with molecular investigations initially targeting the 16S rRNA gene. This chapter provides a review of the predominant geochemical zones of ASC springs including H₂S(aq) degassing and elemental S deposition immediately downstream of spring discharge, followed by oxidation of As^III and Fe^II to form As^v-rich, hydrous ferric oxide (HFO) microbial mats. The distribution of 16S rRNA sequences throughout the outflow channels of ASC springs reveals the potential importance of H₂, H₂S, S^o, As, and Fe-transforming microorganisms that are related to members of the genera Stygiolobus, Caldococcus, Hydrogenobaculum, Metallosphaera, Thiomonas, Acidimicrobium, and Meiothermus, as well as several novel uncultivated bacterial and archaeal clones detected in other thermal habitats. Results to date support the hypothesis that phylogenetic and functional diversity in these geothermal systems is defined by geochemical and temperature regimes throughout the outflow channels.