A modified Flory-Huggins equation accounting for the solvation of polymer molecules by water molecules was used to model the phase behavior of aqueous two-phase systems (ATPS) formed by poly(ethylene glycol) (PEG) and dextran. The parameters of the equation were obtained by fitting experimental equilibrium data either accounting for or disregarding dextran polidispersity. The modified equation was subsequently applied to calculate partition coefficients of biomolecules in these systems. It was found that accounting for polidispersity did not affect significantly the calculated phase equilibrium, but increased the agreement of calculated partition coefficients with experimental data. Further improvement was obtained by using a size dependent interaction parameter for dextran pseudo-components. (C) 2003 Elsevier Ltd. All rights reserved.