A novel experimental approach to study the hygroscopic properties of multi-component inorganic aerosols was demonstrated using a laser trapping technique. The efflorescence and deliquescence phase transitions of the equimolar mixture of NaCl and NaNO3 particles levitated in air were reversibly induced by controlling relative humidity. The two-stage phase transitions of the particles during the dehumidifying and humidifying processes were successfully observed in air. To our knowledge, this is the first experimental result to observe the reversible hygroscopic behavior of single optically-levitated multi-component inorganic aerosols in air. Furthermore, to elucidate the influence of solid substrates on the homogeneous and heterogeneous nucleation processes, the efflorescence relative humidity (ERH) and mutual efflorescence relative humidity (MERH) in air were compared with those observed on a hydrophobic glass substrate. The average ERH and MERH values of the NaCl–NaNO3 particles levitated in air were lower than those obtained for the particles deposited on the hydrophobic glass substrate.