Hydrothermal fluids circulating in Earth’s crust, besides representing a resource of exploitable energy, are responsible for a number of phenomena of scientific and practical interest, including ore deposits, the biological communities surrounding hydrothermal vents near midocean ridges, and the existence of extremophilic microbial life. In this chapter, we examine how multicomponent chemical reaction modeling can be used to better understand the behavior of hydrothermal fluids. We specifically consider the origin of fluorite deposits, the mixing of hydrothermal fluids with ambient seawater at hydrothermal vents associated with black smokers, and the availability of chemical energy to chemolithotrophic life during this mixing process.