The efflorescence and deliquescence processes of Mg(NO3)2 aerosol particles deposited on ZnSe substrate 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 existed 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- NO-3) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg2+ and NO3. The efflorescence and deliquescence processes of Mg (NO3) 2 aerosol particles deposited on ZnSe substrates 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 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 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. into the supersaturated region due to the gradual evaporation of water. Finally, amorph In the FTIR-ATR spectra of the supersaturated Mg (NO3) 2 droplets, the absorbance of the symmetric stretching vibration of NO3- (v1-NO-3) from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg2 + and NO3.