Probe molecule spectroscopy characterizes the surface environment of mixed NaCl/MgSO4 (0.01-50 wt % MgSO4) aerosol particles as a model for marine aerosol. Two complementary measurements, the probe's excited state spectroscopy and photoionization efficiency, measure the electronic properties of the particle surface and monitor phase changes that are driven by changes in relative humidity (RH). The results illustrate that over a wide range of composition, these particles have a layered structure with NaCl in the core and primarily hydrated MgSO4 at the surface. Modeling the spectroscopic data reveals that the surface layer is not a uniform shelf and that the coumarin 314 probe molecules partition selectively to the MgSO4 domains. The surface layer has a pi* value of 1,7, indicative of a very high interfacial polarity. In cases where MgSO4 is a minor component (<= 10 wt %), the NaCl component crystallizes at 44% RH, consistent with the single salt NaCl result. Deliquescence-mode experiments with these particles show that the MgSO4 component forms a solution at 42% RH, prior to the full deliquescence of the particle. For mixed particles with 50 wt % MgSO4, the crystallization of NaCl occurs at 35% RH, and the predeliquescence of MgSO4 occurs at 38% RH owing to the contribution of MgCl2 in the surface layer. A model surfactant, SDS, slightly lowers the RH of the NaCl formation to approximately 42% and leads to the formation of a thin soap film that persists to low values of RH.