A lattice model is presented to compute salt-induced liquid-liquid phase separation in aqueous polymer solutions. The Gibbs energy of mixing contains an electrostatic contribution given by Pitzer's extension of the Debye-Hu ¨ckel function, and the extended Flory-Huggins theory that uses empirical functions of temperature and composition as binary interaction parameters. Our Flory-Huggins theory assumes complete dissociation of salt into ions, but it does not distinguish between cation and anion; our theory represents the water-salt-polymer mixture as an incompressible ternary system consisting of water, ion, and polymer. In the extended Flory-Huggins theory, the binary interaction parameter between water and ion, and that between water and polymer, are obtained by correlating the observed activity of water in each of the two relevant binary systems. The electrostatic contribution does not contain adjustable parameters. We show that the electrostatic contribution to the Gibbs energy of mixing is responsible for inducing salt-polymer aqueous two-phase systems. Calculated phase diagrams are compared with experiment for aqueous solutions containing polyethylene glycol and a single salt at room temperature. The efficiency of a salt to form salt-polymer aqueous two-phase systems is discussed in terms of ion valence and the interaction parameter between ion and polymer.