This work focuses on the hygroscopic behavior of NaCl-NaNO₃ mixed particles as reacted sea-salt aerosol surrogates. The experimental phase diagrams for their deliquescence and efflorescence are determined, and the efflorescence process is explained. This study has high atmospheric implications, because the observed phases and chemical microstructures are expected to help in elucidating the complexity of real ambient sea salt aerosols, their hygroscopic properties, aqueous phase chemistry, etc.