The phase equilibria of two ternary system of Polyethersulfone (PES) - n-methyl pyrolidine (NMP) - distilled Tap water (H2O) and Polysulfone (PSf) - NMP - H2O has been investigated critically using both theoretical and experimental methods. The cloud point measurement technique was used for determination of experimental binodal curves. Gamma-gamma (gamma-gamma), gamma-phi (gamma-phi), phi-gamma (phi-gamma) and phi-phi (phi-phi) approaches were assessed using measured binodal data. Compressible regular solution (CRS), Nonrandom Two Liquids model (NRTL), Universal Quasi-Chemical theory (UNIQUAC) were used for activity coefficient (gamma) calculations and Gasem-Gao-Pan-Robinson modification to the PR EOS (PRGGPR) and Twu-Coon-Cunningham modification to the RK EOS (RKTCC) were used for fugacity coefficient (phi) calculations. The Othmer-Tobias and the Hand correlation equations were used to check the consistency of the obtained tie-line data, by which the distribution coefficients (D) and separation factors (S) were calculated. Description of LLE using CRS-UNIQUAC in a gamma-gamma approach, the RKTCC-RKTCC in a phi-phi approach, CRS-RKTCC in a gamma-phi approach and the RKTCC-UNIQUAC in a phi-gamma approach gives the best agreement and correlation to the experimental data. The overall performance of gamma-gamma approach using CRS-UNIQUAC model revealed an averaged Accumulative Absolute Relative Deviation (AARD) of 2.47%. (C) 2016 Elsevier B.V. All rights reserved.