High signal-to-noise ratio (S/N) Raman spectra of (NH4)(2)SO4 droplets deposited on a quartz substrate were obtained from dilute to supersaturated states upon decreasing the relative humidity (RH). When the molar water-to-solute ratio (WSR) decreases from 16.8 to 3.2, the (nu)1-SO42 2- band changes very little, that is, showing a red-shift of only about 1 cm-(1) (from 979.9 to 978.8 cm-(1)) and an increase of its full width at half-maximum (fwhm) from 8.3 to 9.8 cm-(1). Other vibration modes such as nu(2)- and nu(4)-SO4 (2)- bands appear almost constantly at 452 and 615 cm-1. Such kind of a spectroscopic characteristic is different from previous observation on other cations, indicating that the interactions between S0(4) (2)- and NH4+ in supersaturated states are similar to those between S04 2 and H2O in dilute states. After fitting the Raman spectra with Gaussian functions in the spectral range of 2400-4000 cm- (1), we successfully extracted six components at positions of 2878.7, 3032. 1, 3115.0, 3248.9,.3468.4, and 3628.8 cm- (1), respectively. The first three components are assigned to the second overtone of NH4+ umbrella bending, the combination band of NH4+ umbrella bending and rocking vibrations, and the NH4+ symmetric stretching vibration, while the latter three components are from the strongly, weakly, and slightly hydrogen-bonded components of water molecules, respectively. With a decrease of the RH, the proportion of the strongly hydrogen-bonded components increases, while that of the weakly hydrogen-bonded components decreases in the droplets. The coexistence of strongly, weakly, and slightly hydrogen-bonded water molecules must hint at a similar hydrogen-bonding network of NH4+, SO4 (2)-, and H2O to that of pure liquid water in supersaturated (NH4)(2)SO4 droplets.