Eighteen different poly(ethylene glycol) (PEG)/citrate aqueous two-phase systems at 25.0 degrees C were compared with otherwise identical systems containing 2% by mass sodium chloride. For all systems, the PEG concentration difference between the phases, a parameter proportional to tie-line length, increased with increasing total system PEG concentration. The PEG concentration difference was also greater in each system containing the sodium chloride than in identical systems without sodium chloride. In systems without sodium chloride, the pH difference between the phases increased with increasing PEG concentration difference from about 0.05 to 0.42 pH units. In the systems containing sodium chloride, the pH difference between the phases also increased with increasing PEG concentration difference, but only from 0.08 to 0.20 pH units. Indole 3-acetic acid and tryptamine, a model pair of oppositely charged compounds, were partitioned in the two-phase systems. In each system, partition coefficients were proportional to the concentration difference between the phases, irrespective of the proportion of sodium citrate and citric acid used. In the systems without sodium chloride, the partition coefficients of indole acetic acid were 1.5 to 4 times greater than the partition coefficients of tryptamine. The difference between the partition coefficients of these two solutes was greatest in phase systems with the greatest pH difference between the phases. In the systems containing sodium chloride, the partition coefficients of indole 3-acetic acid were only 1 to 2 times greater than the partition coefficients of tryptamine. The addition of sodium chloride to this phase system reduced the pH difference and concomitantly the difference between the partition coefficients of the oppositely charged solutes.