When a certain water-soluble polymer is dissolved in water together with another kind of hydrophilic polymer or with a certain inorganic salt at specific concentrations, two aqueous phases can be formed. These aqueous two-phase systems or aqueous biphasic systems have been successfully used for the separation of biological materials, such as cells, organelles, enzymes and proteins, because they essentially have a nondenaturing environment. 1,2It has also been shown that the aqueous two-phase partition technique can be efficient for the separation of inorganic compounds 2-5 and small organic molecules. 6 Although a number of different water-soluble polymers may be utilized to form aqueous two-phase systems, polyethylene glycols (PEGs) are mainly used in combination with dextrans or inorganic salts. Because PEGs are nontoxic, nonflammable and nonvolatile the PEG-based aqueous two-phase systems cause less environmental problems compared to conventional solventextraction systems utilizing water-immiscible organic solvents.The partition of solute compounds in aqueous two-phase systems depends on numerous factors originating from the polymers and inorganic salts to be used for forming the two phases. As well as the variables concerning the polymers, the type and concentration of the salts are among the most important factors. It has been demonstrated that the ionic composition in aqueous two-phase systems exerts pronounced effects on the partition of solute compounds, particularly on that of ionic solutes. 4,5,7,8 Johansson 7 has indicated that the partition coefficients of ionic solutes in a PEG-dextran two-phase system depends both on the type of salt added to the system as well as the net charge of the solute. Rogers et al. 5 investigated the partition behavior of pertechnetate ion in PEG-alkali metal and ammonium sulfate salt systems, and revealed that the trends in the partition coefficients follow from the relative salting-out ability of the salts used.However, the effects of the salts added upon the partition of solute compounds are rather complex because the concentrations of the polymers in the coexisting phases depend on the type and concentration of the salts. Zaslavsky et al. 9,10 and Bamberger et al. 11 have shown that the salt additives alter the polymer composition of the coexisting phases in the aqueous two-phase systems formed by two different polymers, such as PEG-dextran and Ficoll-dextran. This means that the ionic composition in the aqueous two-phase systems influences the partition of solutes not only by its own effect, but also by the effect on the polymer composition of the two phases. Therefore, the inherent effect of ions on the solute partitioning in aqueous two-phase systems should be evaluated under the conditions where the effect of the polymer concentrations can be regarded as being constant.In the present study, we chose PEG-salt (Na2SO4, K2HPO4 and Na2HPO4) two-phase systems and investigated the partition bahavior of inorganic cations and anions as a function of the difference in the c...