The hygroscopic properties of sulfate-containing particles are important to understanding the behavior of atmospheric aerosols. At high concentrations, chemical interactions between sulfate ions with the countercations are significant and lead to the formation of contact pairs. In this paper, Raman spectroscopy was used to study the structural changes of single aqueous droplets of equal molar Na2SO4/MgSO4 mixture, Na2SO4, (NH4)(2)SO4, MgSO4, ZnSO4, and CdSO4 in relation to their hygroscopic properties in an electrodynamic balance. The molar water-to-solute ratio (WSR) and the Raman spectra of droplets equilibrated at different ambient relative humidities were measured. When RH is reduced, the WSR of the Na2SO4/MgSO4 droplet decreases from 25.3 to 4.2 without crystallization but with two distinct transition points. The first one appears at WSR = 18.9, where the WSR is more sensitive to RH and the shoulder at 995 cm(-1) of the nu(1)-SO42- band of the Raman spectrum shows an abrupt change in the ratio of the intensity at 995 cm(-1) to that at 984 cm(-1). This 2 WSR is close to the minimum ratio (18) required to support the hexaaquo structures for both Mg2+ and Na+ ions. A mixture of the contact ion pairs of Mg2+ O6-x(SO42-)O-x(H2O)(Na+O6-y)-Na-.(SO42-)O-y(H2O) (y < x < 6), which change the spectral characteristic of the shoulder at 995 cm(-1), is formed. These contact ion pair mixtures share sulfate ions and water molecules, which invalidate empirical mixing rules of water activity of atmospheric aerosols such as the ZSR model. As RH is further reduced, the second transition point appears at WSR 7.7, where the WSR becomes almost insensitive to RH and another shoulder at 1002 cm(-1) appears in the spectrum. The mixture of the contact ion pairs finally evolves into the double salt Of MgSO(4)(.)Na(2)SO(4)(.)4H(2)O, giving two new Raman peaks at 1007 and 1043 cm(-1). Transitions of the hygroscopic properties coinciding with the formation of monodentates were also observed for MgSO4 and ZnSO4 droplets.