Twenty-three in-situ coal sections were sampled in Indiana mines to investigate mercury (Hg) concentrations, with a special reference to in-seam vertical variations in Hg distribution. In addition to raw coal, corresponding float fractions were also analyzed to evaluate the effectiveness of Hg reduction during conventional pre-combustion washing. Hg content in Indiana coals in the locations studied averages 0.11 mg/kg, which after recalculation into equal energy units corresponds to 9.2 lb Hg/1012 Btu (???39.5 kg Hg/ 1010 MJ). Hg content is reduced after washing to 0.07 mg/kg on average, corresponding to an equal energy Hg loading of 5.2 lb Hg/ 1012 Btu (???22.3 kg Hg/1010 MJ). A large portion of Hg appears to be associated with pyrite, as in the Springfield Coal Member of the Petersburg Formation (Pennsylvanian). In the samples from the Danville Coal Member of the Dugger Formation (Pennsylvanian), Hg is associated to a larger extent with organic matter. Large differences in Hg content may exist between different portions of a coal bed in a single location, indicating that very careful, complete sampling is necessary to get representative Hg concentration data. ?? 2005 Elsevier B.V. All rights reserved.