In the course of severe light water reactor (LWR) core melt accidents, the formation and presence of water soluble compounds will affect the behavior of fission products in the primary system and in the containment. A liquid aerosol mixed with an insoluble component has an affinity to stick on surfaces. A relocation of the deposited aerosol may occur depending on the mole fraction of the solid component and the viscosity of the liquid component in the deposited material. In the very humid conditions expected in the containment, steam will condense on the hygroscopic particles, thereby increasing the size of the particles and settling rate. As a first step in modeling the effects of hygroscopicity, the water activity of a CsOH solution was implemented in the condensation model. The model predicts a significant contribution of CsOH hygroscopicity on the suspended mass concentration, which is in accordance with the observations from the latest large-scale containment aerosol experiments. Results of this simplified CsOH hygroscopicity model were compared with aerosol particles consisting of mixed solutions (e.g., CsOH—Cs2CO3—CsI—H2O) expected in particles released during severe accidents. Water activities of binary and mixed solutions were first calculated using semiempirical methods and these results were compared with the available experimental data. Secondly, different heat and mass transfer models were compared to find a suitable method for the growth rate calculations of hygroscopic aerosol particles. We can conclude that sedimentation of hygroscopic aerosols is an effective removal mechanism for airborne fission products at a high relative humidity in the LWR containment during severe core melt accidents.