The partitioning of bovine serum albumin and ovalbumin in different two-phase aqueous polymer systems is investigated using a thermodynamic approach. Systems used were polyethylene glycols (PEGs) of molecular weights 1000 to 10,000 Da and Dextran T500 (500,000 Da). Ovalbumin transfer to the top phase is exothermic, which suggests an electrostatic interaction between the hydroxyl groups of PEG and the hydrophilic side chain of the protein, whereas the bovine serum albumin partition is an endothermic process that is entropically driven, which coincides with its high surface hydrophobicity. The effect of PEG molecular weight on enthalpy and heat capacity changes, associated with the partition of both proteins, is examined on the basis of a preferential interaction of low-molecular-weight PEG with the protein surface. (C) 2001 John Wiley & Sons, Inc.