Thr emissions of trace elements and mercury from stationary combustion sources are determined by the occurrence of these elements in fuels, the transformation of the elements into vapor and particles in furnaces, and the ability of the vapors and particles to penetrate the air pollution control devices (APCDs). For electrostatic precipitators (ESPs), in use at greater than 90% of coal-fired utility boilers in the US, the preferential escape of particles is in the 0.1-1.0 mu m size. The major source of particles in this size range is the vaporization and condensation of the inorganic constituents in the parent fuel, Potentially toxic elements, although mainly confined to the particulate phase, may therefore show enhanced release to the environment as a result of preferential condensation on the surface of submicron particulate matter. Surface reaction with larger fly ash particles can reduce these emissions by redistributing the trace elements away from the difficult-to-capture submicron particulate. In contrast, mercury is the most volatile of the trace elements in coal and its escape is near complete when the mercury is in the elemental vapor form. Mercury emissions may be mitigated, however, by transformation to mercuric chloride, more readily captured either in scrubbers or by collection in the particulate form. In this paper, we present our recent research developments contributing to an improved understanding of the relative importance of factors determining trace element emissions. Significant progress that has been made in understanding and quantifying each of the processes governing the transformation of the inorganic constituents of fuels leading to promising results on the prediction of emissions given detailed characterization of fuels, the combustion conditions to which they are exposed, and the characteristics of APCDs. (C) 2000 Elsevier Science B.V, All rights reserved.