The interaction of water vapor with submicron NaCl condensation aerosols has been investigated with a tandem differential mobility analyzer (TDMA) and compared to experiments with NaCl dispersion and PbS condensation aerosols. The mobility equivalent diameters of the NaCl condensation aerosol particles decreased by up to 50% when exposed to relative humidities (RH) below the deliquescence threshold. The extent of shrinking was determined by relative humidity, initial particle size (19-200 nm) and aerosol generation parameters. Above similar to 75% RH hygroscopic growth was observed in agreement with Kohler theory. Dynamic shape factors for the NaCl condensation aerosol particles before and after humidification were calculated, indicating a transformation from sparse branched-chain to more compact agglomerates. The investigated PbS particles shrank much less but still significantly when exposed to water vapor, which is discussed in View of the microstructural rearrangement processes at work. (C) 2000 Elsevier Science Ltd. All rights reserved.