Mercury associated with fly ash is a significant contaminant released in flue gas emissions from coal-fixed power plants. This work focuses on the association of Hg with other elements and phases as well as the molecular-level speciation of Hg in bulk and <0.1 mu m sized fly ash particles reacted with a Hg-containing simulated flue gas stream. Following reaction under conditions chosen to simulate an electrostatic precipitator operating at 140 degrees C, fly ash (bulk and <= 0.1 mu m) from a Kentucky power plant was analyzed using microscopic and spectroscopic techniques. The <= 0.1 mu m fraction dominates Hg uptake, with total Hg concentrations increasing from 100 ppb to 610 ppm after reaction, whereas bulk ash concentrations increase from 11 to 164 ppb. Synchrotron-based micro-X-ray fluorescence mapping of the reacted <= 0.1 mu m fraction showed that Hg is present in two major regions: Fe-rich areas and Hg hot spots not associated with Fe. X-ray absorption spectroscopic analysis revealed that Hg is associated with Br and Cl, is bound to iron oxides, and occurs as HgS (cinnabar). Fourier transform infrared (FTIR) spectroscopic analysis of the fine fraction revealed carboxylic, alcoholic, and alkane functional groups. Density functional theory simulation of the vibrational frequencies of a carboxylic group bonded to Hg reproduced the same frequency shifts and peak intensity reductions (relative to carboxylic acid alone) observed in the experimental FTIR spectrum of the Hg-reacted fly ash fine fraction, suggesting Hg(II) binding to organic matter. Our results reveal complex interactions between Hg and coal fly ash in the combustion stream that produces less bioavailable forms of Hg than He present in the unreacted flue gas. Such information is critical for safe disposal of Hg-containing fly ash in landfills or use in cementitious products.