On-line systems for monitoring mercury in liquid and gaseous production and waste streams have been developed, utilising atomic fluorescence spectrometry (AFS) as the basis for detection. Instrumentation has been designed for unattended continuous operation. Laboratory chemistries for the vapour generation of mercury, normally performed off-line in batch mode, have been adapted and optimised for continuous, flow-injection analysis for varying sample types and chemical forms of mercury. The system has typical analysis cycle of 7 minutes, a limit of detection of 10 pg ml ˉ¹ a linear range up to 100 µg ml ˉ¹ and has been applied in industrial environments for the continuous monitoring of mercury in incineration wastewater and sulphuric acid. The system was validated by on-site trials for periods of one week, during which time comparative off-line laboratory measurements showed good agreement. An automated system for monitoring mercury in natural gas streams has also been developed and validated by laboratory and on-site industrial trials. A heated pressure let-down system was designed in order to facilitate sampling of high-pressure gas streams without condensation of heavier fractions. The heated sampling line was interfaced with an automated system for trapping mercury, from variable volumes of gas, onto gold amalgamation traps, with subsequent desorbtion and analysis by AFS. The method detection limit for a 58 litre sample of natural gas was 30 pg m ˉ³ which was sufficient to determine residual mercury in natural gas streams even after mercury scrubbing had been performed. The system was validated by laboratory trials and spiking experiments during on-site trials at a gas processing facility, which resulted in complete installation and commissioning.