Three new phase diagrams for polymer-salt aqueous two-phase systems (ATPS) at 23 degrees C are presented. Then, partition coefficients for a series of five dinitrophenylated amino acids were obtained experimentally in five polymer-salt ATPS. The ATPS were obtained combining a polymer, polyethylene glycol (PEG) or Ucon (a random copolymer of 50 % ethylene oxide and 50 % propylene oxide), and a salt (Na2SO4, (NH4)(2)SO4, or Li2SO4). The series of amino acids allowed the calculation of the free energy of transfer of a methylene group, which is used as a measure of the relative hydrophobicity of the equilibrium phases. Ucon-rich phases proved to have a higher affinity for methylene groups than PEG which suggests a more efficient biphasic separation system with lower cross-contamination.