Knowledge about the chemical speciation of Hg(II) is a prerequisite for a proper understanding of biogeochemical processes in control of the transformation of Hg(II) into toxic and bioaccumulating monomethyl mercury. Of critical importance are structures and the stability of Hg(II)-complexes with inorganic and organic sulfur ligands in aqueous and solid phases of soils and sediments. On the basis of Hg L(III)-edge EXAFS experiments, we report Hg(II) to form a four-coordinated metacinnabar [beta-HgS(s)] phase when reacted with disordered FeS(s) (mackinawite), at pH 9.0 and a Hg(II) to FeS(s) molar ratio of 0.002-0.012. When Hg(II) (1000-20,000 microg Hg g(-1)) was added to mixtures of <5 days of aged FeS(s) (2-20%) and an organic soil at pH 5.7-6.1, a mixture of Hg(II) coordinated with two organic thiols [Hg(SR)(2)] and Hg(II) coordinated with four inorganic sulfides in a metacinnabar-like phase was formed. Surface complex formation between Hg(II) and FeS(s), or substitution of Hg(II) for Fe(II) in FeS(s), was not observed. Quantities of beta-HgS(s) and Hg(SR)(2) formed (as determined by EXAFS) were in fair agreement with theoretical thermodynamic calculations, as described by the reaction: Hg(SR)(2) + FeS(s) = HgS(s) + Fe(2+) + 2RS(-). The calculated stability constant for this reaction (log K = -16.1 - -15.4) supports a strong bonding of Hg(II) to organic thiols, corresponding to a log beta(2) for the formation of Hg(SR)(2) on the order of 42 or greater.