Oxygen isotope exchange between BaSO 4 and H 2O from 110 to 350°C was studied using 1 m H 2SO 4-1 m NaCl and 1 m NaCl solutions to recrystallize the barite. The slow exchange rate (only 7% exchange after 1 yr at 110°C and 91% exchange after 22 days at 350°C in 1 m NaCl solution) prompted the use of the partial equilibrium technique. However, runs at 300 and 350°C were checked by complete exchange experiments. The temperature calibration curve for the isotope exchange is calculated giving most weight to the high temperature runs where the partial equilibrium technique can be tested. Oxygen isotope fractionation factors (α) in 1 m NaCl solution (110–350°C), assuming a value of 1.0407 for α CO 2 H 2 O at 25°C, are: 10 31 nα BaSO 4−1 m NaCl = 2.64 ( 10 6 T 2 ) − 5.3 ± 0.3 . These data, when corrected for ion hydration effects in solution ( Truesdell, 1974), give the fractionation factors in pure water: 10 31 nα BaSO 4 H 2O = 3.01 (10 6/T 2) −7.3 ± 0.1 . In the 1 m H 2SO 4-1 m NaCl runs, sulfur isotope fractionation between HSO − 4 and BaSO 4 is less than the detection limit of 0.4%. A barite-sulfide geothermometer is obtained by combining HSO − 4H 2S and sulfide-H 2S calibration data. Barite in the Derbyshire ore field, U.K., appears to have precipitated in isotopic equilibrium with water and sulfur in the ore fluid at temperatures less than 150°C. At the Tui Mine, New Zealand, the barite-water geothermometer indicates temperatures of late stage mineralization in the range 100–200°C. A temperature of 350 ± 20° C is obtained from the barite-pyrite geothermometer at the Yauricocha copper deposit, Peru, and oxygen isotope analyses of the barite are consistent with a magmatic origin for the ore fluids.