The hygroscopic behavior of CH3SO3Na, CH3SO3NH4, (CH3SO3)(2)Mg, and (CH3SO3)(2)Ca particles as a function of relative humidity (RH) has been studied using microscopic Fourier transform infrared (micro-FTIR) spectroscopy. The approach used exposure of substrate-deposited, similar to 1 mu m dry-size particles to humidified nitrogen followed by micro-FTIR spectroscopy over a selected sample area. The results show that CH3SO3Na particles undergo characteristic phase transitions at deliquescence relative humidity (DRH) of 71% and efflorescence relative humidity (ERH) of similar to 40%. In contrast, CH3SO3NH4, (CH3SO3)(2)Mg, and (CH3SO3)(2)Ca particles do not undergo phase transitions and exhibit continuous, reversible uptake and evaporation of water under the influence of changing RH. The extent of water uptake is quantified and presented as water-to-solute ratios (WSR) in particles as a function of RH. The WSR values are determined from the