Thermodynamic analysis has shown that particles dried from multicomponent aqueous aerosols do not have a homogeneous chemical morphology except at the eutonic point, The particles are composed of a pure salt core surrounded by a mixed salt coating, where the core composition is solely determined by the original aerosol composition but the coating is identical in chemical composition to the eutonic point and is independent of the original component mole fractions. Rapid Single-particle Mass Spectrometry (RSMS) has been used to probe the chemical composition of particles dried from KCl/NaCl, KCl/KI, and (NH4)(2)SO4/NH4NO3 mixed solutions at different mole ratios. By measuring the peak area ratios corresponding to the intensity of certain ions in the spectrum, the chemical composition of the surface layer can be inferred. The results suggest that the surface layer is enriched with the minor component and are consistent with our thermodynamic predictions. The significance of the dried particle morphology to its mass transport characteristics is discussed.