Over 750 detailed chemical analyses of fluids, collected from 30 geothermal wells over 12 years of operation, were accumulated at Cerro Prieto, Mexico. These results are used as a case study to establish the physical and chemical processes taking place and to construct a geochemical model of the resource. A knowledge of these processes is a prerequisite for conducting successful isotope studies. The following regularities and correlations were observed in the geothermal wells of Cerro Prieto. 1. 1. The concentrations of Cl, Na, K, Li, Ca, B, HCO 3 and SiO 2 in individual wells vary by factors of up to 2. (A common range for Cl is, for example, 10,000–20,000 mg/l). A similar range is observed when the maximum ion contents of all the wells are compared. 2. 2. A positive correlation is observed between the Cl content and the concentrations of Na, K, Li, Ca, B and SiO 2 in individual wells and throughout the field. Comparison of all the wells also revealed positive correlations of the concentrations of Cs, Rb and Br. Patterns (1) and (2) are due to concentration-dilution processes, such as steam loss or mixing with condensed steam and possibly fresher water from a shallow aquifer. 3. 3. In most wells a consistently lower Cl content (about 10,000 mg/l) is observed. This value suggests the salinity of the deep-seated brine, all higher values being caused by steam losses. These steam losses seem to occur both in the aquifer and in the wells. 4. 4. The plots of the various ions vs Cl reveal a conservative pattern in the data of the individual wells (i.e. best-fit lines extrapolate to zero). This indicates that no significant chemical reactions occur during the ascent of the fluids in the wells. In contrast, K and SiO 2 plots of the different wells reveal a reactive pattern both when plotted against the Cl content and enthalpy. Hence, these ions are reactive in the aquifers, responding to temperature zonations. This pattern makes the K (or Na/K) and the SiO 2 values useful as geothermometers in the Cerro Prieto system. 5. 5. The above regularities are best ascribed to one type of brine existing at Cerro Prieto, with a Cl content of around 10,000 mg/l, and a temperature zonation. The variety in the observed samples is caused by steam losses (partially caused by production conditions such as diameter of the well orifice) and by K and SiO 7 equilibrations at different temperatures in the geothermal system.