The deliquescence behavior of multicomponent aerosols has been studied extensively by measuring the particle size or weight growth at controlled relative humidities. To our knowledge there has been no experimental investigation of the chemical composition of the aerosol surface as a function of relative humidity, which is extremely important in understanding a number of issues in atmospheric chemistry and physics. Rapid single-particle mass spectrometry (RSMS) has been used in this work to explore the deliquescence behavior of particles generated from NaCl/KCl, NaCl/NaNO3, and (NH4)(2)SO4/NH4NO3 mixed solutions and then conditioned at different relative humidities. Thermodynamic predictions of the crystallization/deliquescence behavior of each mixture were also given. For NaCl/KCl and NaCl/NaNO3 mixtures, the data were generally consistent with thermodynamic analysis and earlier investigations. The results indicate that as the ambient relative humidity reaches the deliquescence point of the system, the chemical composition of the particle surface changes gradually with increasing relative humidity until the particle transforms completely to an aqueous droplet. The observed deliquescence began at relative humidities somewhat lower than that predicted by equilibrium thermodynamics. This is probably due to the cracks on the particle surface. The deliquescence behavior of (NH4)(2)SO4/NH4NO3 mixtures observed was very complicated and was generally not consistent with thermodynamic predictions.