Using mixing rules in electrolyte models for estimating the thermodynamic properties of multicomponent mixtures requires knowledge of data for each binary solution. Very often, a ternary solution in a single-particle levitation experiment can achieve a higher ionic strength as compared with the ionic strengths derived from any of the binary solutions alone. Using water activity data of ternary solutions and the Zdanovskii-Stokes-Robinson and the Kusik and Meissner (KM) equations, water activities of binary solutions of (NH4)2SO4, NaCl, and NH4Cl at supersaturations higher than any previously reported are calculated. With available data from the literature on aqueous supersaturated droplets of NH4NO3-(NH4)2SO4 ternary solutions and letovicite at ambient temperatures, water activities (>0.32) of (NH4)2SO4 solutions for ionic strengths up to 112 mol kg-1 are obtained. The new water activity relation enables the use of the KM and Pitzer models at higher ionic strengths than in previous research. Applying the same method to the data on (NH4)2SO4-NaCl and NH4NO3-NaCl systems yields water activities (>0.28) of NaCl solutions for ionic strengths up to 25 mol kg-1. Similarly, water activities (>0.34) of NH4Cl solutions up to 42 mol kg-1 are calculated from the measured data on NH4Cl-NH4NO3 and NH4Cl-(NH4)2SO4 systems. New water activity measurements for the NH4Cl-NH4NO3, NH4Cl-(NH4)2SO4, and NH4NO3-NaCl systems are also reported. ¿ 1999 American Geophysical Union