A calibration assembly generates elemental mercury and oxidized mercury for calibrating components of a mercury monitoring system, including making necessary adjustments to efficiencies of a mercury compound converter and an elemental mercury detector. The calibrator generates an elemental mercury sample having a known elemental mercury concentration, [Hg.sup.0].sub.1 and combines an oxidizing component with the elemental mercury sample, thereby producing a reduced concentration of elemental mercury [Hg.sup.0].sub.2 within the sample. The calibrator measures the concentration of elemental mercury [Hg.sup.0].sub.2 within the sample and calculates a difference between the known elemental mercury concentration, [Hg.sup.0].sub.1 and the reduced concentration [Hg.sup.0].sub.2. The difference between [Hg.sup.0].sub.1 and [Hg.sup.0].sub.2 is substantially equal to the concentration of oxidized mercury produced by the calibrator. The concentration of oxidized mercury can also be calculated as the difference between elemental mercury detected after passing a sample stream containing oxidized mercury through a channel which includes the converter and the reduced concentration [Hg.sup.0].sub.2 detected after passing the sample stream through a channel which bypasses the converter. By providing oxidized mercury at calculated concentrations, the calibrator allows a user to calibrate continuous emission monitoring systems for accurate response to both elemental mercury and oxidized mercury.