Uric acid (UA) is an important component in biological matrices, and the development of new methods for extracting/separating UA from several complex matrices is necessary. A viable alternative is the use of an aqueous two-phase system (ATPS), which is an environmentally safe and efficient technique. In this work, an extensive study of the thermodynamic approach of UA partitioning was carried out in an ATPS formed with a polymer, sulfate salts, and water. Initially, the new ATPS formed with polyethylene glycol (400 g mol(-1)), lithium sulfate, and water was characterized by obtaining the position of the binodal curves and the phase compositions. The components' segregation increases with the increase in the concentration of the polymer and salt where the top phase (TP) becomes richer in polymer and poorer in electrolyte, and the bottom phase has the inverse behavior. In the range of the pH studied, pH 2.40, 5.40, and 6.60 showed no effect on the binodal curve position and phase compositions, while the temperature (288.15, 298.15, and 308.15 K) evaluation indicated that the phase separation process was entropically driven. Afterward, a study of UA partitioning was carried out in several ATPSs, evaluating the effect of system composition, pH, temperature, and ATPS-forming components on the partition coefficient (K) of the UA. The K values ranged from 1.03 +/- 0.04 to 6.05 +/- 0.25, indicating a partition preference for the TP for all tie-line length (TLL) values. Furthermore, it is noted that the increase in TLL caused an increase in K, which decreases with increasing the temperature; that is, the partition of uric acid is temperature-dependent, and the phase transfer process of the UA is exothermic. The pH effect study showed that the ionized form of UA has a greater interaction with the components of the TP than that of the molecular form because the K value at pH 6.60 (K = 7.59 +/- 0.23) is higher than at pH 2.40 (K = 1.98 +/- 0.21), while at pH 5.40 (K = 3.84 +/- 0.13), the value is intermediate. This behavior is due to the strong electrostatic interaction between the pseudopolycation, formed by Li+ ions plus polyethylene glycol in TP and the ionized form of UA. Finally, higher K values were obtained for the system formed with polyethylene glycol (400 g mol(-1)), lithium sulfate, and water. Thus, the balance of interactions between the system components and UA is the driving force that will drive the partition in the ATPS.