This thermodynamic model describes the phase equilibria of mixtures of electrolytes and organic species in aqueous solutions existing as aerosol particles. The activity coefficient of each species in solution is explicitly related to the chemical composition by treating the (inorganic) ion-water, organic-water and ion-organic interactions with a combined Pitzer-UNIFAC thermodynamic approach. It was parameterized with a new type of multifunctional "meta-group" to better represent measured propel-ties of long-chain monofunctional compounds and short-chain multifunctional compounds. Interactions between dissolved electrolytes and organic species are modeled using the "salting-out" effect of NaCl with organic compounds to predict the hygroscopic growth of particles composed of a salt and diacid. The predicted growth agrees well with laboratory measurements. The presence of 50% malonic acid decreases the growth of pure (NH4)(2)SO4 by 20% at high relative humidities, while mixtures with 50% succinic acid and 50% glutaric acid cause decreases of 35% and 38%, respectively. The mixing of organic compounds with solubility higher than 4 mol.L-1 with salt can decrease the observed DRH. The mixture of malonic acid and (NH4)(2)SO4 is predicted to start taking up water at 58%, much lower than the DRH of pure (NH4)(2)SO4 (80%). Insoluble compounds do not change the observed DRH, while reducing the amount of water taken up. The predicted water contents for internal and external mixtures are largely similar, with small differences predicted for mixtures of soluble organic species. The largest deviation of 10% between the water contents of internal and external mixtures occurs for 50% malonic acid with 50% (NH4)(2)SO4. For less soluble compounds such as succinic acid and glutaric acid, the two types of growth generally agree within 3%.