Abstract Liquid–liquid equilibrium data of aqueous two-phase systems (ATPSs) composed of the thermo-sensitive copolymer L31 and (NH 4 ) 2 SO 4 at 288.15 K, L31 and (NH 4 ) 2 SO 4 /Na 2 SO 4 /Li 2 SO 4 /Na 2 C 4 H 4 O 6 at 293.15 K were determined, respectively. The Othmer–Tobias and Bancroft equations were successfully used to correlate the tie-line data of the investigated systems. In the L31–(NH 4 ) 2 SO 4 ATPS, it was found that the size of the two-phase region of phase diagrams increased with an increase in temperature. Meanwhile, the salting out ability of cations followed the order: Na + > NH 4 + > Li + , and the salting out ability of SO 4 2− is stronger than C 4 H 4 O 6 2− . It was found that evaluating the salting outing ability of ions by using the Gibbs energy of hydration or Hofmeister series is not available for all of ATPSs.