A large amount of mercury has been discharged on the U.S. Department of Energy's Oak Ridge Site (Tennessee) as a part of the U.S. nuclear weapons program during the 1950s through the early 1960s. Increases in mercury concentration in fish and in lower East Fork Poplar Creek of Oak Ridge have been recently reported. The objectives of this initial study were to investigate mercury speciation and also bioavailability in Oak Ridge-contaminated soil. Results show that the HgCl2 complex is the predominant species in soil solution while organically bound mercury is the major solid-phase fraction in soils contaminated with soluble mercury compounds. Mercury uptake by plant roots correlates well with concentrations of mercury complexes in soil solution, while Langmuir relationships are found between mercury concentrations in plant shoots and in soil solid-phase components. Mercury in HgS-contaminated soils is to some extent bioavailable to plants. Mercury transformation occurs from more labile fractions into more stable fractions. Volatilization is a major pathway of the natural attenuation of Oak Ridge soil contaminated with soluble mercury compounds.