An aqueous two-phase system (ATPS) composed of PEG35000, Na2CO3, K2CO3, and their mixtures at 298 K was studied. The liquid–liquid equilibrium (LLE) of these systems, including binodal curves, tie-lines, tie-line length, and slope of the tie-line, were obtained. Additionally, for the first time, salt mixtures with different initial mass ratios of 1:3, 1:1, and 3:1 were used to prepare the aqueous two-phase systems. The effect of electrolyte and salting-out power for these systems was examined and compared. Consistent with the literature, it was found that the salting-out power of Na+ is higher than that of the K+ cation. Furthermore, in Na2CO3 and K2CO3 mixtures, increasing the amount of sodium ions resulted in stronger salting-out power. The LLE data was correlated with the Othmer- Tobias, Bancroft, and Setschenow models, and good agreement was found with all three models.
