As part of its program to provide technology for reducing undesirable environmental impacts of mineral processing operations, the Bureau of Mines developed the citrate process for desulfurizing industrial stack gases that contain from 0.1 to 2 vol pct so2. In this regenerable flue gas desulfurization process, so2 is absorbed from cleaned and cooled industrial flue gas with a citric acid- sodium citrate solution. The absorbed so2 is subsequently reacted with h2s to precipitate marketable elemental sulfur and regenerate the solution for recycle. Laboratory investigations of process parameters provided information for designing larger scale continuous operations. Information gathered from operation of two pilot plants, one at a copper smelter and one at a lead smelter, confirmed the laboratory results. Data were obtained for design and construction of a commerical-sized demonstration plant at the George F. Weaton power station, Monaca, Pennsylvania. A preliminary operation of the plant was demonstrated under a cost-sharing Bureau-industry contract. This bulletin describes the citrate process and gives results of the laboratory, pilot plant, and demonstration plant studies, as well as corrosion investigations.