The partitioning of nonionic compounds in the PEG/salt ATPS is predominantly determined by the difference in the concentration of PEG between the upper and lower phases, ΔwPEG, regardless of the type and concentration of the phase-forming salt. On the contrary, the distribution of ionic solutes depends on the type of the phase-forming salt even though the values of ΔwPEG are identical. The inherent effect of the background phase-forming cations and anions on the distribution of ionic solutes in polymer/salt ATPS must be evaluated under the conditions where the effect of the polymer concentration can be regarded as being constant. This chapter discusses the effects of counterion and co-ion of the phase-forming electrolyte on the distribution of ionic solutes based on the experimental results obtained for the PEG/salt ATPSs formed by PEG and Na2SO4, Na2HPO4, or K2HPO4. The abilities of K+ and SO42− to salt out the polymer are lower than those of Na+ and HPO42−, respectively, but the former ions are more effective than the latter ions in extracting oppositely charged ions into the PEG-rich phase when compared under constant ΔwPEG. On the other hand, Na+ and HPO42− are more effective than K+ and SO42− for the extraction of the ions which have the same charge sign as theirs. It will be shown that these effects of the background phase-forming ions on the distribution coefficient of an ionic solute can be interpreted by the stoichiometric ion partition model.