The efflorescence and deliquescence processes of Mg(NO3)2 aerosol particles deposited on ZnSe sub-strata have been investigated through in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) technique at the molecular level.At relative humidity (RH) of ~3%, Mg(NO3)2 particles ex-isted as amorphous states.The amorphous Mg(NO3)2 particles were transformed into crystalline Mg(NO3)2·nH2O (n≤5) with slight increasing of RH. Thermodynamically stable Mg(NO3)2·6H2O crystals were gradually formed on the particle surface and started to be dissolved at the saturation point (~53% RH). At the same time, a continuous phase transition from Mg(NO3)2.nH2O (n≤5) to Mg(NO3)2·6H2O occurred on the particle surface.This led the solid particles to completely deliquesce at 76% RH, which was much higher than the saturation point of 53% RH.In the efflorescence process, Mg(NO3)2 droplets entered into the supersaturated region due to the gradual evaporation of water.Finally, amorphous particles were formed when RH decreased below 5%.In the FTIR-ATR spectra of the supersaturated Mg(NO3)2 droplets, the absorbance of the symmetric stretching vibration of NO3- (v1-NO3-) clearly be-came stronger.It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg2+ and NO3-.