The phase equilibrium of CsCl-polyethylene glycol (PEG) 1000 - H2O, solid-liquid equilibria (SLE), and liquid-liquid equilibria (LLE) data for aqueous two - phase system (ATPS) containing CsCl and PEG4000/6000 was determined at 298.15 K. The equilibrium phase compositions, binodal curves, tie-line length (TLL), and slope of tie-line (STL) for the aqueous two phase systems were determined. Furthermore, three non-linear expressions developed by Mistry, Hu, and Jayapal were used to correlate the binodal data, the effective excluded volumes (EEV) were calculated from binodal curve data, the tie-line data were correlated with Othmer-Tobias and Bancroft equations. It was found that the salting out effect is greater with larger molecular mass of PEG as the EEV of PEG6000 is larger than PEG4000. As the molecular mass of PEG increased, the binodal curve shifted toward the origin, which increased the liquid-liquid biphase region. (C) 2019 Elsevier Ltd.