Quantum mechanical methods are used to determine the structures, stabilities, and properties of a number of different mercury(II) sulfide, bisulfide, and hydroxide species in aqueous solution. Relativistic effective core potential bases and methods ranging in rigor from Hartree-Fock to quadratic configuration interaction with single and double substitutions are used for the gas-phase calculations while explicit solvation with small numbers of water molecules, SCRF, and IPCM methods are used to describe hydration. We find that the species with molecular composition HgS is unstable in water solution and that it probably exists as Hg(SH)(OH), hydrated strongly by four waters. Its isomer HgS(H2O) is unstable, spontaneously rearranging to Hg(SH)(OH). By comparing the dissociation energetics of Hg(SH)(OH) with that of other compounds containing a -SH group, we have determined that Hg(SH)(OH) has a pK(a) of 7 or larger, so that it is not significantly deprotonated near neutral pH. The HgS and HgS(H2O) species are also unstable to photolysis by sunlight since their lowest energy allowed spectral transitions occur in the visible or near UV. By contrast, Hg(SH)(OH) does not photolyze in sunlight. When the SH- concentration is increased, Hg(SH)(OH) becomes unstable with respect to Hg(SH)(2)(OH)(-1). Therefore, the species which is observed to partition into organic solvents under conditions of low SH- concentration is actually Hg(SH)(OH) and the species produced at higher SH-concentrations, which does not partition into organic solvents, is Hg(SH)(2)(OH)(-1).