Abstract In Chapter 2 chemical data from a set of geothermal wells were used to derive an empirical geothermometer based on silica content. In this section we shall derive this geothermometer by an independent method based on experimental data and examine some other empirical geothermometers which are related to mineral equilibria in the system. During exploration the chemical geothermometers provide rare and valuable windows into the deep system through which we see "as through a glass darkly” (St Paul), the choice and interpretation of geothermometer data being the art of the exploration geochemist.