The fate of the mercury content of four UK coals has been investigated using a 1 MWth Combustion Test Facility (CTF). The results demonstrate that the mercury retention in dust is related to the carbon content of the dust, with increasing retention as carbon levels increase. Conversely, as carbon levels decrease, vapour phase mercury emissions increase. The measurements also showed that at a given carbon-in-dust level, mercury retention increases as flue gas temperature decreases. A short series of speciation measurements indicated that approximately 80% of the vapour phase mercury was present in the oxidised (Hg2+) form. No relationship was apparent between mercury retention and coal chlorine content over the range considered (0.11-0.44%), In addition to the rig-based studies, a short series of measurements on two different 500 MW pf-fired boilers showed the same relationship between carbon-in-dust and mercury retention, with increasing mercury in dust levels as excess air levels were reduced and carbon-in-dust levels increased. (C) 2000 Elsevier Science B.V. All rights reserved.