The release of mercury front flue gas desulfurization (FGD) gypsum in the manufacture of 100% FGD wallboard was Studied. Calcination, with product temperatures ranging from 150 to 180 degrees C, appears to he the most likely process to release mercury front the FGD. Two samples evaluated in trials conducted to simulate the calcining of FGD in flash calcining and kettle process showed that the potential of mercury release exists. The differing amounts of mercury released by the two processes could he separated h orders of magnitude, Measurements in real production facilities are essential to developing a valid answer to the question of mercury release during, FGD gypsum wallboard production. (c) 2004 Elsevier Ltd. All rights reserved.