(Liquid + liquid) equilibrium (LLE) for the {poly ethylene glycol di-methyl ether 2000 (PEGDME(2000)) + Na2HPO4 + H2O) system have been determined experimentally at T = (298.15, 308.15, 313.15, and 318.15) K. The effects of temperature on the binodals and tie-lines for the investigated aqueous two-phase system (ATPS) have been studied. An empirical non-linear expression developed by Merchuk was used for reproducing the experimental binodal data. In this work, the three fitting parameters of the Merchuk equation were obtained with the temperature dependence expressed in the linear form with (T - T-0) K as a variable. Furthermore, the modified local composition segment-based NRTL and Wilson models and also osmotic virial equation were used to describe the LLE data of the studied system. Also, the effects of the type of salt on LLE are discussed. In addition, the effects of end groups of the polymers PEGDME(2000) and poly ethylene glycol 2000 on phase forming ability were studied. The complete phase diagram for the poly ethylene {glycol di-methyl ether 2000 (PEGDME(2000))+ Na2HPO4 + H2O) system has also been determined at T = 298.15 K. (C) 2010 Published by Elsevier Ltd.