This is the first of two papers that review approximately 200 publications on mercury (Hg) in coal; topics reviewed in thispaper include: (a) analytical and historical introduction; (b) some peculiarities of Hg environmental geochemistly; (c) estimation of the Hg coal Clarke value; (d) occurrence of high-Hg coals; (e) mode of Hg occurrence in coal; (f) factors influencing the distribution of Hg in coal; and (g) origins of Hg in coal. Aspects of the environmental impact of Hg due to coal combustion and coking are noted throughout the review, and are more fully examined in second and final paper of this review. The Clarke value for Hg in coal (world-wide average Hg content) is 0.10 (+/- 0.01) ppm Hg, regardless of coal rank. However, if the Hg content is expressed on an ash basis, bituminous rank coals average 0.87 (+/- 0.08 ppm) Hg, and lower rank coals average 0.62 (+/- 0.06) ppm. In general, Hg is a very '' coalphile '' element; that is, it has a strong affinity for organic and inorganic coal matter, but is obligatory authigenic. There are some coals that are enriched in Hg by 1 to 2 orders of magnitude above the Clarke value. Such coals are located in the Donbas (Ukraine), Appalachian basin and Texas (USA), Russian Far East, and Southern China. Mercury in these coals is usually accompanied by other chalcophile trace elements. In low-sulfur coals, which are generally poor in Hg, only two Hg sites dominate: Hg-org and Hg-sulfide (commonly pyritic mercury, Hg-pyr). High-sulfur coals usually contain higher amounts of Hg, which is typically in the Hg-pyr form. In the unique Hg-enriched Donbas coals, both HgS and metallic mercury (Hg degrees) also occur. Because of the extremely strong affinity of Hg2+ or humic matter, it is theoretically possible for syngenetic (or early epigenetic) Hg to accumulate in peats and lignites. Indeed, the amount of Hg in some tropic and moderate zone peats is greater than or equal to the Hg content of typical coal. Until the recent discovery of syngenetic volcanic Hg accumulations in Canadian coals, such Hg accumulations in coal were unknown. The most important type of Hg enrichment in coals is an epigenetic, low-temperature hydrothermal process; the Nikitovka (Ukraine), Warrior (USA), and Guizhou (China) basins are good examples. In the two latter coal basins, the paragenetic association Hg-As-Au-Tl is similar to Carlin-type An deposits. The most notably Hg-rich basin is the Donbas-Donetsk coal basin situated in the Ukraine and Russia. Numerous coalfields in the Donbas are enriched in Hg by one order of magnitude above the coal Clarke level. The Hg in the Donbas was due to hydrothermal mineralization passing through sheared zones. The extraction and use of Hg-bearing Donbas coals have created very difficult environmental problems for the Ukrainian people. (c) 2004 Elsevier B.V. All rights reserved.