When balancing the element mercury (Hg) two coal-frred power plant units one with slag tap boilers (ST, 2 x 220 MW) and one with a dry bottom boiler (DB, 475 MW) were compared. Both systems are provided with electrostatic precipitators (ESP), nitrogen OJddes removal (DeNOx) and flue gas desulfurization (FGD) systems. The Hg in the flue gas is predominantly in gas phase. Only 15 o/o ofthe Hg introduced by the coalleaves the unit with the bottom or fly ash. Depending on the operating mode, 30 to 40 o/o of the Hg is separated in the FGD systems. The overall separation rate for the total system ranges between 45 to 55 o/o, the residue is emitted in the fonn of gaseous Hg species. At fullload, the Hg concentration in the cleaned gas is less than 6 JJ.g/m3 2>. In the flue gas path of another dry bottom boiler (DBl, 480 MW) the concentrations ofthe gaseous species of bivalent mercury (Hlf+), eiemental mercury (Hg0), and total mercury content (E Hg) were determined. The sum of the concentrations of H!f+ and Hg0 is in agreement with the measurement of E Hg. Directly downstream of the boiler H!f+ dominates with 77 %, while Hg0 amounts to 23 %. In the high-dust DeNOx system Hg0 is oxidized almost completely to Hg2+ (96 o/o ). Air heater and electrostatic precipitator do not influence the Hg species concentrations. The FGD system eliminates approximately 80 o/o of the H!f+. At the same time the quantity of Hg0 increases by the factor 10. In the cleaned gas Hg0 dominates with 76 o/o as compared to Hg2+ with 24 %. At fullload the concentration ofE Hg in the cleaned gas is also below 6 JJ.g/m3.