The polyethylene glycol-sodium sulfate aqueous two-phase system has been characterized at 23 degrees C. Tie-lines for the phase diagram were obtained experimentally. Phases in equilibrium were characterized by means of the solvatochromic parameters pi*, alpha, and beta, which provide a measurement of the polarity/polarizability and the H-bond donor and acceptor abilities, respectively. The ability of the phases to participate in hydrophobic interactions was characterized by means of the free energy of transfer of a methylene group between the conjugated phases, using the partition of a homologous series of dinitrophenylated amino acids. The results show the effect of the presence of polymer and salt in the aqueous phase, and a comparison of both phases with pure water is made.