Wellhead and downhole water samples were collected and analyzed from a 114.3-m well at Coso Hot Springs (Coso No. 1) and a 1477-m well (CGEH No. 1) 3.2 km to the west. The same chloride concentration is present in hot waters entering both wells (about 2350 mg/kg), indicating that a hot-water-dominated geothermal system is present. The maximum measured temperatures are 142¿C in the Coso No. 1 well and 195¿C in the CGEH No. 1 well. Cation and sulfate isotope geothermometers indicate that the reservoir feeding water to the Coso Hot Spring well has a temperature of about 240¿--250¿C, and the reservoir feeding the CGEH well has a temperature of about 205¿C. The variation in the chemical composition of water from the two wells suggests a model in which water-rock chemical equilibrium is maintained as a convecting solution cools from about 245¿C to 205¿C by conductive heat loss.