Extensive water activity, density, and refractive index data at 25 ¿C are reported for mixed-salt solutions, NaCl-KCl, NaCl-NaNO3, NaCl-Na2SO4, Na2SO4-NaNO3, and (NH4)2SO4-Na2SO4. The data are obtained from hydration experiments using the single-particle levitation technique developed recently for measuring the thermodynamic and optical properties of microdroplets. These data, covering the whole concentration range from dilute solutions to high supersaturations, provide an opportunity to explore the light-scattering properties of both internal and external mixtures of the chloride, sulfate, and nitrate aerosols of atmospheric importance. It is shown that for sulfate and nitrate aerosols as solution droplets, the light-scattering properties do not differ appreciably among all mixture types and compositions, as long as the dry-salt aerosols have the same particle-size distribution. However, for mixed-salt aerosols containing NaCl, the light-scattering properties do depend upon the composition and particle-size distribution, although not so much on the mixture type.