INTRODUCTION Until 1966 the method of Stiff and Davis(1) for predicting the tendency of oil field water to deposit calcium sulfate was used extensively by Atlantic. Richfield Co. This predictive method correlates well with field observations in brines of low ionic strength or those composed primarily of sodium chloride. However@ in the case of brines containing excessive common ion and/or magnesium@ the correlation diminishes as the concentration of these ions increases. This has been recognized by Atlantic Richfield and various individuals using the Stiff and Davis predictive method. (2@ 3@ 4) Therefore@ a more exact@ yet simple@ predictive technique was sought. Since 1966 Atlantic Richfield has calculated calcium sulfate solubility in brine by the classical thermodynamic method using available solubility data from the literature. More recently@ these calculations have been made using solubility data produced in this laboratory. These measurements were made under carefully controlled conditions specifically to provide solubility product constants throughout the range of temperature and ionic strength common in oil field operations. To simplify calculations@ a program has been written utilizing time sharing computer service.