New experiments have been conducted to determine the speciation of dissolved mercury (Hg) over wide pH (1–12) and sulfide concentration ranges (0.5–30 mM) and in the presence of elemental sulfur (S⁰) or Hg⁰, conditions that encompass those of near-bottom and pore waters of sediments. Samples containing synthetic red mercuric sulfide (HgS, cinnabar), buffer solution, aliquots of bisulfide (HS^–1) solution, and, in special cases, S⁰ or Hg⁰ were prepared anaerobically and allowed to equilibrate for several months. Filtered samples were analyzed for pH, total sulfide (S^2–), and total mercury [Hg]_tot. Plots of [Hg]_tot values vs. pH at varying S^2– verified the formation of three previously known mercury-sulfide complexes (HgS₂H_n ^n–2) and revealed that a new Hg₂SOH⁺ complex is important at low pH and low S^2–. Our constants for ionic strength (I) 0.7 and 25⁰ C are as follows: K₁=10^{–5.76(+0.71, –1.02)} for HgS_cinn+H₂S HgS₂H₂ ⁰; K₂=10^{–4.82(+0.72, –1.10)} for HgS_cinn+HS^– HgS₂H^–; K₃=10^{–13.41(+0.76, –0.93)} for HgS_cinn+HS^– HgS₂ ^2–+H⁺; K₄=10^{–8.36(+0.71, –0.93)} for 2HgS_cinn+H⁺+H₂O Hg₂SOH⁺+H₂S. With decreasing pH, below 1, Hg solubility decreased sharply, indicating the formation of a new solid phase, inferred to be corderoite (Hg₃S₂Cl₂). From our solubility data, we calculated the free energy of formation (G_f ^o) of Hg₃S₂Cl₂ to be –396 (+3, –11) kJ/mol. In experiments where excess S⁰(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K₅=10^{–1.99(+0.69, –1.27)} for 2HgS_cinn+2HS^– + nS⁰ Hg₃S₄ ^IIS_n ^oH₂ ^2–. Data from experiments where Hg⁰(aq) was added confirmed the reversibility of HgS dissolution. An application of our mercury-sulfide speciation model to a natural anoxic basin, Saanich Inlet, British Columbia, is discussed.