A thermodynamic model for the computation of phase and chemical equilibrium in aqueous systems based on minimization of the Gibbs energy is developed using new thermochemical data and Fitter's activity coefficient model. New data for the thermodynamic standard properties Delta(f)G(o), Delta(f)H(o) and C-p(o), are determined from our experimental results, which are based on the spectrometric in situ analysis of both the vapor and the liquid phase. The studies have been made in the concentration range pertinent to flue gas purification processes: p(So2) between 0.01 and 1 kPa, c(HCl) up to 1 mol dm(-3), C-H2SO4 and c(CaCl2) up to 0.5 mol dm(-3). The temperature varies from 298 to 333 K. In comparison with earlier data, a better correlation of important flue-gas/water subsystems like SO2 + water is achieved and a formerly proposed complexation of sulfur dioxide in the SO2 + HCl + H2O system is confirmed. In particular, it is shown that on this basis the phase equilibria of SO2 + CaCl2 + H2O and SO2 + HCl + H2SO4 + H2O can be predicted without further adjustment of parameters.