In this work the interfacial properties of aqueous two-phase systems (ATPS) consisting of an aqueous solution of polyethylene glycol (PEG) and dextran with different molar masses were investigated. To calculate the interfacial properties the density gradient theory was combined with the lattice cluster theory and the Wertheim association theory. With help of this thermodynamic model the liquid-liquid equilibria of the investigated ATPS were calculated in a molar mass range of PEG from 4000 to 20,000 g/mol and of dextran from 10,000 to 500,000 g/mol; whereas the thermodynamic model was fitted to the phase equilibrium in several selected ATPS. Phase equilibria in all other systems were predicted in a good accordance with experimental data. On the basis of the modelled phase equilibria the interfacial tension was calculated by the density gradient theory in an excellent agreement with experimental data. Moreover, the concentration profile of the phase forming components across the interface was calculated and it could be seen that there is no accumulation of any component in the interface and that the length of interfacial region depends on PEG molar mass. (C) 2015 Elsevier B.V. All rights reserved.