Investigating　the　hygroscopic　behavior　of　haze　aerosol　particles　is　essential　for　understanding　their　physicochemical　properties　and　their　impacts　on　regional　weather　and　visibility.　An　environmental　scanning　electron　microscope　equipped　with　a　home-made　transmission-scattering　electron　imaging　setup　and　an　energy　dispersive　spectrometer　was　used　for　in-situ　observations　of　pure　water-soluble　(WS)　salts　and　Beijing　haze　particles.　This　imaging　setup　showed　obvious　advantages　for　improving　the　resolution　and　acquiring　internal　information　of　mixed　particles　in　hydrated　environments.　We　measured　the　deliquescence　relative　humidity　of　pure　NaCl,　NH4NO3,　and　(NH4)(2)SO4　by　deliquescence　crystallization　processes　with　an　accuracy　of　up　to　0.3%　RH.　The　mixed　haze　particles　showed　hygroscopic　activation　like　water　uptake　and　morphological　changes　when　they　included　WS　components　such　as　nitrates,　sulfates,　halides,　ammoniums,　and　alkali　metal　salts.　In　addition,　the　hygroscopic　behavior　provides　complementary　information　for　analyzing　possible　phases　in　mixed　haze　particles.