The Babcock and Wilcox (B&W) and McDermott Technology (MTI, currently the B&W Research and Development group) have had a continuing program over the past decade for characterizing and optimizing mercury control in flue gas desulfurization (FGD) systems. These efforts have led to the characterization of mercury emissions control at two utility installations and full-scale demonstration (55 and 1300 MW) of the effect of a mercury control performance enhancement additive for wet FGD systems. This paper presents the results of the mercury emissions control testing conducted at these two sites. The performance is related to EPA Information Collection Request (ICR) data from an FGD system supplier's perspective, highlighting the need to consider the effects of system design and operation when evaluating mercury emissions control performance. (C) 2004 Published by Elsevier B.V.