The thermodynamics of mercury-sulfur-chlorine-water systems are summarised in the form of potential-pH and activity-pH diagrams. The potential-pH diagram for Hg-Cl-H2O shows that chloride ions greatly extend the areas of stability of Hg-2(2+) and Hg2+ ions, forming Hg2Cl2 (c) and HgCl42- ions. While the potential-pH diagram for the Hg-S-H2O system predicts oxidation of HgS to S(VI) and elemental Hg, this was not observed experimentally with bulk and particulate HgS, due to the strong interaction of sulfur with mercury, and the large activation energy involved. Hence, metastable potential-pH diagrams were calculated, which provide a good prediction of the observed behaviour of HgS and Hg in sulfide solutions. Voltammetry and chronoamperometry were used to investigate the oxidation and reduction behaviour of an HgS electrode. A novel process for the treatment of mercury-bearing effluents is described, based on the indirect reduction of HgS precipitates using electrogenerated Cr(II) as a reductant.