In spite of the modicum of laboratory work on two-phase separation (TPS), the mechanism of the process is still far from being fully understood. Two-phase distribution of polyethylene glycol 6000 (PEG6K) in buffer pH 7.4 (ionic strength, 1.2 – 2.0 mol dm-3) and polyethylene (glycol) 70000 (PEG70K) in buffer pH 7.4 (ionic strength, 0.2 – 1.2 mol dm-3) were determined in the temperature range between 15 and 40oC. Concentration range of PEG6K used was between 150 and 270 g/dm3, and that of PEG70K was between 10 and 50 g/dm3. The phase separation of PEG70K was characterized by liquid-solid phase distribution whose apparent equilibrium constant increases with increasing concentration of PEG70K at fixed ionic strength. That of PEG6K was characterized by liquid-liquid phase separation of which the complexity of the profile of the dependence of logarithm of equilibrium constant of phase separation on concentration of PEG6K reduces with increasing ionic strength and temperature. At low ionic strength and temperature the profile has a maximum at ca. 207 g/dm3 and a minimum at ca. 240 g/dm3. The natural logarithm of distribution constant of PEG70K increases with increasing concentration of PEG70K, transforming from concave-down profile at low temperature and ionic strength, to concave-up profile at high ionic strength and high temperature. The finding of this work show that PEG6K is more sensitive to change in concentration of electrolyte per unit polymer than PEG70K. Optimum aqueous two phase separation is achievable by raising the concentration of the electrolyte as the concentration of the polymer is raised in solution.