Ammonium and sodium are important cations found in atmospheric aerosols. While the water activities of sodium salts and their mixtures have been studied by many researchers, data for ammonium mixtures are rather limited because of the volatility of ammonium chloride and ammonium nitrate. In this research, water activities of ternary solutions of NH4Cl-NH4NO3, NH4Cl-(NH4)2SO4, NaCl-NH4Cl, NaNO3-NH4NO3, NaCl-NH4NO3, and NaNO3-(NH4)2SO4 of single levitated particles from dilute concentrations to high supersaturations are measured using an electrodynamic balance (EDB). On the basis of a well-calibrated change of relative humidity (RH) into the EDB, water activity data of a droplet at various concentrations at atmospheric pressure were measured in less than 1 hour. The evaporation loss of NH4Cl and/or NH4NO3 is negligible in this short experimental period at room temperature and atmospheric pressure. The measured results are compared with data from available literature and used to evaluate the performance of four commonly used thermodynamic models: the Zdanovskii-Stokes-Robinson (ZSR), Kusik and Meissner (KM), and Pitzer models and aerosol inorganic model (AIM). Predictions made using these models arc, in general, consistent with our experimental data. In terms of the total mass fraction of solute (mfs) as a function of water activity (aw), the average standard deviations between the predictions of the models and the measured data are 0.005 for ZSR, 0.011 for KM, 0.016 for Pitzer, and 0.012 for AIM. The ZSR model is recommended for estimating the water activities of Na+-NH4+ salt mixtures because of the high accuracy of its prediction and the wide range of water activities to which it can be applied. Complete crystallization resulting in dry solid particles was observed in the ternary solutions of NH4Cl-(NH4)2SO4 and NaCl-NH4Cl. Partial crystallization was observed in the solutions of NaCl-NH4NO3 and NaNO3-(NH4)2SO4, in which case the droplets continue to release water as RH decreases after crystallization. A notable exception was the NaNO3-(NH4)2SO4 of mole ratio 3:1, where the mfs decreases as RH decreases. The AIM was used to predict the possible phase compositions of these partially crystallized particles. Further research is needed to understand the hygroscopic properties and phase compositions of partially crystallized multicomponent aerosols at low relative humidities. ¿ 2000 American Geophysical Union