Trona (Na 3HCO 3CO 3•2H 2O) injection has been used to control SO 2 emissions in coal-fired power plants. This study evaluated the impact of trona injection on fly ash physical and chemical characteristics, especially the leaching characteristics. One set of paired coal fly ash samples, collected from a full-scale power plant with and without trona injection for SO 2 control, was used. The results showed that trona injection introduced significant amounts of soluble sodium compounds into the fly ash, elevated the fly ash pH, reduced the ash surface area, and changed the fly ash morphology. A comparison of 30 constituents in leachates from both ashes demonstrated that trona ash leached significantly greater amounts of Na, sulfate, chloride, fluoride, total inorganic carbon (TIC), and oxyanions, especially As and Se. Effects of the liquid/solid (L/S) ratio, pH, dry storage time, and leaching time on As and Se leaching and speciation were examined. As V, Se IV, and Se VI were the As and Se species in trona ash leachates. With an increase in storage time and leaching time, As speciation was unchanged but the ratio of Se VI/Se IV significantly increased. For the control sample, both As III and As V were detected in the short-term leachates but As III was converted to less leachable As V within several days of leaching, and Se speciation changed from only Se IV to only Se VI after 3 years of dry storage. © 2011 American Chemical Society.