Natural gas often contains mercury at concentrations that vary from below 1 to above 10000 μg/m3. Mercury is both toxic and potentially damaging: it can corrode or embrittle common gas plant components. Gas plants reduce mercury in natural gas with mercury removal units (MRUs). MRUs use fixed bed absorbers, often with sulfur-impregnated carbon or other chemisorb- ents as the active material. The Mercury Monitoring System is an ideal tool to determine the efficiency of each MRU in real time, and necessary to suc- cessfully monitor and control mercury concentrations during natural gas production and processing.