Aqueous two-phase systems (ATPSs) have found wide application in the separation of biological molecules such as amino acids, peptides and proteins. Most of the research in this area has been dedicated to experimental determination and less effort has been devoted to the proper thermodynamic modeling of these systems. This work presents a new model for ATPS that accounts for solvation effects between polymer and water molecules, which are commonly reported to be responsible for phase separation. The model uses the Flory-Huggins equation as a starting point and modifies the combinatorial term by considering the presence of a hydration shell. The modified equation parameters were fit to a number of isotherms of poly(ethylene glycol) - dextran systems, and the results obtained are reported herein. These results show that the adopted modification leads to remarkable improvements in the performance of the model.