Supersaturated MgSO4 aerosols and dilute MgSO4 solutions were studied by FTIR spectroscopic techniques (i.e., aerosol flow tube (AFT) and attenuated total reflection (ATR)). The hygroscopic properties Of MgSO4 aerosols were investigated with results in good agreement with previous measurements by a scanning electrodynamic balance (SEDB). Well-defined spectral evolutions with changing relative humidity (RH) for the v(3) band Of SO42- and the water O-H stretching envelope could be directly related to the observed hygroscopic properties Of MgSO4 aerosols. When the RH decreased from similar to 55 to similar to 40%, the V, band of SO42- in supersaturated MgSO4 aerosols was observed to transform from a sharp peak at similar to 983 cm(-1) into a wide band at similar to 1005 cm(-1). The sharp peak at -983 cm(-1) was mainly assigned to such associated complexes of Mg-2+ and SO42- as double solvent-separated ion pairs (2SIPs), solvent-shared ion pairs (SIPs), and simple contact ion pairs (CIPs) in supersaturated MgSO4 aerosols, while the wide band at similar to 1005 cm(-1) was due to polymeric CIPs chains, probably the main component of gels formed in MeSO4 aerosols at low RHs. Relating to this v, band transformation, the peak position of the v(3) band was first shown to be a sensitive indicator of CIPs formation, spanning across similar to 40 cm(-1) on the formation of polymeric CIPs chains, which could also be supported by aerosol composition analysis in the form of water-to-solute molar ratios (WSR). In the water O-H stretching envelope, the absorbance intensities at 3371 and 3251 cm(-1) were selected to represent contributions from weak and strong hydrogen bonds, respectively. The absorbance intensity ratio changing with RH of 3371 to 3251 cm(-1) could be related to the previous observations with the v(1) and v(3) bands of SO42-. As a result, the formation of CIPs with various structures in large amounts was supposed to significantly weaken hydrogen bonds in supersaturated MgSO4 aerosols, while 2SIPs and SIPs were not expected to have similar effects even when occurring in abundance. In comparison with MgSO4 aerosols, the peak positions of the v(3) band of SO42- in artificial seawater aerosols implied that the MgSO4 component should be contained as gels or concentrated solutions in the fissures of microcrystals of sea salts for freshly formed seawater aerosols at low RHs.