The Western Kentucky University mobile laboratory for monitoring mercury emissions measured the mercury levels in a 100-MW boiler with wall-fired low-NOx burners. Mercury emissions were monitored while burning seven coals, using semicontinuous emission monitoring at the air preheater outlet and electrostatic precipitator outlet. The collected data was then scaled and analyzed using stepwise regression analysis. The results showed that initial mercury concentration in the coal, as well as chlorine and sulfur levels, all influence the amount of mercury emissions. After mercury content, chlorine had the major role in the levels of vapor-phase mercury present in the flue gas. Chlorine promotes the chemisorption of mercury onto fly ash. Sulfur was shown to be a major factor in the oxidation of elemental mercury but inhibited the adsorption of oxidized mercury onto the fly ash. Further experimental results suggest that both HCl and SO2 may participate directly in the mercury oxidation mechanism.