The objective of groundwater geothermometry is to use a groundwater’s chemistry to estimate the temperature at which it was last in equilibrium with its environment, to better understand the water’s origins. In this chapter, we explore the use of multicomponent chemical reaction modeling as a tool in geothermometry. We first construct a synthetic example to illustrate the calculation process. We then apply the modeling to better understand the chemistry of a hot spring in Iceland as well as geothermal water from that country.