A system for the on-stream X-ray diffractometric determination of mineral phases in slurries was developed, in response to the mining industry＇s need for process control during the beneficiation of industrial minerals. The instrumentation for, and results from testwork conducted at a phosphate rock beneficiation plant in Florida are discussed. Results show that the concentrations of the minerals apatite (var. francolite) and quartz can be accurately determined every few minutes, so that adjustments to the flotation circuit can be executed from the control room when needed. The on-stream measurements can ultimately be integrated into an automated plant control system. Estimates of savings on reagent and manpower costs, as well as the benefits from consistent grades and greater recoveries, have shown that the investment in such a system could be recovered in less than a year.