The partitioning of salts in aqueous two-phase systems formed from polyethylene glycol, dextran, and water produces an electrostatic potential difference between the phases which influences the partitioning of proteins and other biomolecules. In this paper we study the partitioning of, separately, NaCl, NaH2PO4, and NaHSO4 in such systems, as well as the resulting electrostatic potential differences. Polymer concentrations were measured by liquid chromatography and ion concentrations by atomic absorption or inductively coupled emission spectroscopy. A combination of the UNIQUAC, Debye-Hückel, and Brønsted-Guggenheim equations, after accounting for differences in the standard states, resulted in a thermodynamic model that correlates the concentrations of all species (polymers, ions, and water) and allows calculation of the resulting electrostatic potential difference. At comparable concentrations, NaHSO4 produces the largest electrostatic potential difference between the phases and NaCl the smallest.