With impending regulation of mercury from coal-fired power plants, it is important to concentrate efforts on the most mature retrofit control technologies. Injection of powdered activated carbon (PAC) has been deemed the most mature technology, but has until recently only been demonstrated in bench- and pilot-scale experiments. This paper presents some of the results from a field evaluation program of sorbent injection upstream of existing particulate control devices (PCDs). An important component of the field demonstration program is to characterize the fly ash that results from injection of PAC upstream of a baghouse or electrostatic precipitator (ESP). Leaching analyses were performed on fly ash collected during two different field demonstrations; one fly ash was derived from bituminous coal and the other from a subbituminous coal. Several widely accepted leaching methods were used in order to assess the stability of the ash by-product in landfill situations; other ash characterization tests were also used in the study. Little or no detectable Hg was leached as a result of the various leaching protocols. The subbituminous ash conformed to the ASTM C-618 standard for Class C fly ash, but did not pass the Foam Index Test that is also required for sale of this ash for use in concrete formulation. (C) 2004 Elsevier B.V. All rights reserved.