Several studies have shown that aqueous two-phase extraction using an aqueous two-phase system (ATPS) is a promising purification technology for biomolecules (e.g., monoclonal antibodies). To beneficially influence the partitioning of the target biomolecule, a displacement agent can be added to the ATPS. Within this work, the influence of the displacement agents, sodium chloride (NaCl) and lithium bromide (LiBr), on the phase behavior of two ATPSs containing triammonium citrate and polyethylene glycol 2000 (PEG2000) and tripotassium citrate and PEG2000 was determined and compared to previous measurements for trisodium citrate-PEG2000 ATPS. Therefore, the tie-line length (TLL), tie-line slope (TLS), and the size of the biphasic region of the ATPS were investigated at 298.15 K, 101.3 kPa, and pH 7. The results show that the addition of NaCl to the ATPS increases the TLL up to 47.4%, while the TLS stays constant. In contrast, the addition of LiBr causes a decrease in the TLS up to 34.5%, while the TLL remains constant. These results are in agreement with previous measurements performed for trisodium citrate-PEG2000 ATPS. Prospectively, the investigations enable a mechanistic understanding of the displacement agent effect on the phase behavior and transfer of this information to other organic salt-polymer ATPSs to improve their extraction performance.