Under conditions of supersaturation, atmosphericaerosols may form a metastable composition, whichcannot be elucidated by measuring the solubility of thebulk-phase solution, nor can it be predicted by atraditional thermodynamic model. This work employedGC-TCD to measure the deliquescence characteristics ofsingle-salt aerosols and mixed salt aerosols with differentmolar ratios. The GC-TCD method was validated byshowing the deviation of DRH and CRH to be under1%, with an average error ofo5% between eachmeasurement by GC-TCD and the correspondingmeasurement from other literature. The DRH ofNa2SO4aerosol measured by GC-TCD is 84% RH,which agrees with the result of Tang et al. (1995). Theresult significantly differs from the DRH of 93% ofNa2SO410H2O, predicted by the bulk-phase thermo-dynamic model, because the metastable state of ambientNa2SO4aerosol does not form Na2SO410H2O. TheMDRH of NaCl–Na2SO4is found to be 74%, while thatof NH4NO3–NH4Cl is 54%. The RH for forminghomogeneous droplets due to complete deliquescenceof mixed aerosols, is governed by the original mixingratios of the mixed aerosols. The RH for forminghomogeneous droplets of NaCl–Na2SO4is affected bythe mole fraction of Na2SO4in the mixed aerosol,leading to a relatively large difference between themeasured values and the values predicted by thethermodynamic model. Model predictions concerningthe formation of metastable solids are identical tocorresponding single particle measurements for themixed aerosol of NH4NO3–NH4Cl