The effect of temperature variation on isotope fractionation has been studied in rock samples from a well in the Salton Sea geothermal area. A vertical profile of oxygen isotopic composition of whole rock and separated minerals shows extensive exchange between hydrothermal solutions and the country rock. Carbonates and silicates have reached isotopic equilibrium at temperatures as low as 150 degrees and possibly down to 100 degrees C. In contrast to calcite, detrital quartz is remarkably resistant to isotopic exchange at temperatures as high as 340 degrees C. For the main Salton Sea geothermal reservoir, at depths of about 3000 to 6000 feet, the volumes of water and rock that have undergone exchange are roughly equal.