The behavior of mercury under conditions similar to a wet flue gas cleaning process has been discussed in terms of thermodynamic equilibrium. The spectrophotometric technique developed in our laboratory has been used to study the systems HgCl2 + HCl + H2O, HgCl2 + CaCl2 + H2O, and SO2 + HgCl2 + HCl + H2O at 50 degrees C and atmospheric pressure in the absence of oxygen, A thermodynamic model presented in earlier communications is developed further. The substance-specific data required were extracted from literature and used without any further adjustment. The experimental results show that the soluble bivalent mercury (HgCl2) is completely reduced to elemental insoluble form through sulfur dioxide, even in the presence of 0.1 M HCl (i.e., also in strongly acidic solutions). This is also confirmed by the calculations done with the thermodynamic model.