Aiming at gathering further evidence about the molecular-level mechanisms of the polymer-polymer aqueous biphasic systems (ABSs), systematic studies on the vapor-liquid and liquid-liquid equilibria of several ternary aqueous solutions of two water soluble polymers capable or not of inducing phase separation were performed at 298.15 K. It was found that the hydrophilicity of the investigated polymers follows the order PEGs approximate to PEGDMEs > PVP >> PPG400 and the aqueous solutions of PPG400 form ABS with PEG, PEGDME, and PVP however other investigated systems don't form ABS. The ability of PPG400-polymer solutions to form ABS increases by increasing temperature and polymer molar mass. It was also found that, the constant water activity lines of the ternary systems which capable and not of inducing phase separation respectively have the concave (or plane) and convex slopes. Therefore, from the slopes of the isopiestic constant water activity lines one can predict the phase forming abilities of the corresponding systems. (C) 2015 Elsevier Ltd. All rights reserved.