The electrolyte theory of Pitzer and coworkers is used to estimate geometric mean-ion activity coefficients in strong electrolyte solutions at 25–55°C. By using Pitzer's equations with ion-pair parameters estimated in this paper, one can calculate activity coefficients to moderately high ionic strengths in aqueous mixtures containing the following ions: Na+, K+, Mg2+, Ca2+, Cl−, ClO, ClO, HCO, HSO, CO, SO, SO, and S2O. Mixtures containing these ions are present in CaO/CaSO4 flue gas desulfurization units and in laboratory experiments aimed at understanding the chemistry occurring in such units. The computed activity coefficients are relatively insensitive to the values of the estimated quantities. The basis of the estimates, as well as their effect on calculated activity coefficients, is discussed. It is concluded that activity coefficients are estimated with an accuracy of ±25% or better.