Phase diagrams of aqueous two-phase systems (ATPS) composed of polyethylene glycol 20000 (PEG 20000) + polyoxyethylene octyl phenyl ether (Triton X-100) or polyoxyethylene sorbitan monooleate (Tween 80)+ water are determined at 273.15, 293.15 and 313.15K, respectively. The liquid-liquid equilibrium (LLE) experimental data are correlated using the Flory-Huggins model derived from the lattice theories, and new interaction parameters (lambda(12) lambda(13) and lambda 23) between any two compositions (the polymer(1) and the nonionic surfactant(2), the solvent(3)) in the ATPS are estimated at various temperature. Two kinds of fitting results are obtained when the surfactant molecules are regarded as single molecules or micellar molecules, respectively. It indicates that the interaction parameters (lambda(12)) between PEG and nonionic surfactant including Triton X-100 and Tween 80 decrease 52.3% and 105% as single molecules, while decrease 421% and 403% as micellar molecules by raising temperature, respectively. Moreover, the slightly better fitting effect can be observed if the surfactant molecules are regarded as the micellar molecules. The standard deviation between prediction values and experimental data of the components weight fraction is lower than 0.1% using the surfactant micellar parameters, showing the good descriptive quality and applicability of the Flory-Huggins model. (C) 2015 Elsevier B.V. All rights reserved.