Statistical Associating Fluid Theory coupled with Restricted Primitive Model (SAFT1-RPM) previously proposed for representing single-salt solutions in water is extended to multiple-salt solutions using a mixing rule for the hydrated diameter. A binary adjustable parameter in this mixing rule, for a pair of salts, is obtained from experimental osmotic coefficients of the corresponding ternary system. The ternary systems considered contain water and all pairs of NaCl, KCl, NaBr, and KBr. LiCl+NaCl and LiCl+KCl pairs are also correlated. The adjustable parameters are used to predict the density of the ternary systems with or without common ions, the solubility of two ternary systems, and the os-motic coefficient of a quaternary NaCl-KCl-LiCl-H₂O solution.