Parametric experiments were carried out to study the interactions of mercury, SO3, and injected activated carbon (AC) in a coal flue gas stream. The levels of SO3 vapor in flue gas were altered by individually varying flue gas temperature, moisture, or sodium fume injection in the flue gas. Meanwhile, mercury emissions with AC injection (ACI) upstream of an electrostatic precipitator (ESP) were evaluated under varied SO3 concentrations. SO3 measurements using a condensation method indicated that low temperature, high moisture content, and sodium fume injection in flue gas shifted SO3 partitioning from the vapor to particulate phase, subsequently improving mercury capture with ACI. 0.08 g/m(3) of DARCO (R) Hg-LH injection only provided approximately 20% mercury reduction across the ESP in a bituminous coal flue gas containing 28 ppm SO3, but mercury capture was increased to 80% when the SO3 vapor concentration was lowered less than 2 ppm. Experimental data clearly demonstrate that elevated SO3 vapor is the key factor that impedes mercury adsorption on AC, mainly because SO3 directly competes against mercury for the same binding sites and overwhelmingly consumes all binding sites. (c) 2011 Elsevier Ltd. All rights reserved.