Phase diagram data at 4ยฐC was determined for the aqueous two-phase systems composed of polyethylene glycol, dextran, and water. The Flory-Huggins theory of polymer thermodynamics was used to correlate partitioning of biomolecules in these aqueous two-phase systems resulting in a simple linear relationship between the natural logarithm of the partition coefficient and the concentration of polymers in the two phases. This relationship was verified by partitioning a series of dipeptides which differ from one another by the addition of a CH2 group on the c-terminal amino acid residue and by utilizing 8 set of low-molecular-weight proteins. The slope of the line could be expressed in terms of the interactions of the biomolecule with the phase forming polymers and water. The main result for the dipeptides was that knowledge of the partition coefficient in any of the PEWdextraniwater systems, regardless of polymer molecular weight, enabled prediction of the coefficient in all of the systems. The dipeptides were also used for determination of the Gibbs free energy of transfer of a CH2 group between the phases. This quantity was correlated with polymer concentration, thus establishing a hydrophobicity profile for the PEG/ dextradwater systems. The methodology for predicting dipeptide p a r t i t i o n coefficients was extended t o proteins, where it was found that low-molecular-weight proteins gave a linear relationship with the tie line compositions of a phase diagram.