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Hygroscopicity of atmospheric aerosol particles results in changes of the optical properties and chemical composition of the particles with significant effects on visibility degradation, atmospheric chemistry and possibly the global energy balance. Ambient aerosol, sampled in Riverside, California, USA, was characterized by measuring its light scattering coefficient as a function of controlled relative humidity and thermal treatment. These measurements and results from simultaneous filter samples have been compared with data from laboratory measurements and thermodynamic models for the (NH4)2SO4-Na2SO4-H2O system. Results from field experiments have indicated deliquescence humidities in the 74–79% range, after chemically neutralizing the aerosol with NH3 and preheating the sample to 190 °C. Preheating was used to remove more volatile material from the aerosol particles, such as NH4NO3 which could inhibit the occurrence of deliquescence. The measured deliquescence humidity values were less than expected for pure (NH4)2SO4, but their mean is, at the 95% confidence level, the same with the means of predictions for the deliquescence humidities determined from the phase diagram and thermodynamic modeling for the (NH4)2SO4-Na2SO4-H2O system. These results indicate first the importance of mixtures when examining the hygroscopic characteristics of atmospheric aerosol particles, and second the usefulness of in situ optical methods for the assessment of the chemical composition and hygroscopic properties of atmospheric aerosols.DOI: 10.1034/j.1600-0889.1994.00001.x