The activity coefficients of HSO3- and H+ were determined in concentrated aqueous solutions of NaCl in order to provide a theoretical basis for using sea water as a solvent in flue gas desulfurization processes. The activity coefficient of HSO3- was determined al 313 K and 323 ii by measuring the pH of mixtures of NaCl, H2O and small amounts of NaHSO3 at a known partial pressure of SO2. The activity coefficient of H+ was determined over a temperature range of 296 Ii - 333 K by measuring the pH of mixtures of NaCl, H2O and small amounts of HCl. As the ionic strength increased, the activity coefficient of HSO3- decreased while that of H+ increased. The Fitter Equation could correlate the activity coefficients of both species with reasonable accuracies for practical use.