The infrared extinction spectra of model tropospheric aerosols, (NH4)2SO4, NH4HSO4, NaCl, and artificial seawater, have been measured as a function of relative humidity. Experimentally, submicron-sized aerosol particles are spectroscopically monitored as they flow at atmospheric pressure on a 30-s timescale through a room temperature infrared absorption cell. By monitoring absorption features due to either constituent ions or water molecules, we infer both the physical phase and, to some degree, the chemical composition of the aerosol particles. It is observed that (1) solid (NH4)2SO4 and NaCl aerosol particles exhibit deliquescence at 79¿1% and 75¿1% relative humidity, respectively, very close to their thermodynamic values; (2) (NH4)2SO4 and NaCl liquid particles exhibit crystallization at relative humidities of 33¿2% and 43¿2%, respectively, well below their deliquescence points; (3) NH4HSO4 aqueous aerosol particles remain in the liquid state to relative humidities as low as 2%, far below the thermodynamic deliquescence humidity of 39%; and (4) artificial seawater aerosol particles show strong H2O absorption features at low relative humidities, arising either because the particle has not crystallized or because solid hydrates of Mg2+ salts have formed. These observations illustrate the extent to which water will be present in the aerosol condensed phase in both laboratory experiments and in the atmosphere. Specifically, for (NH4)2SO4 and NaCl particles, the water content is expected to be low at relative humidities below the crystallization point, whereas the aerosol particles will be liquid at higher relative humidities. NH4HSO4 and artificial seawater aerosols will both contain significant quantities of water down to very low relative humidities, present either as a liquid or possibly as hydrates of Mg2+ in the case of artificial seawater. By adding gas-phase D2O to NH4HSO4 and artificial seawater aerosols at low relative humidity, condensed phase D2O features appear in the spectra, indicating facile exchange of water between the gas-phase and the particles. Conversely, aerosols with low water content, such as solid NaCl, do not exhibit condensed-phase D2O features in the presence of gas-phase D2O.¿ 1997 American Geophysical Union