Aqueous biphasic systems (ABSs) composed of poly(ethylene glycol) (PEG) and salt have been examined as potential environmentally benign solvents for liquid/liquid extraction. These systems might also represent an alternative to traditional solvent/water systems used in quantitative structure-activity relationships (QSARs). For the application and design of these systems, it is important to have a thorough understanding of the nature of the solvent and its interactions with the solute, and thus, PEG/salt ABSs have been characterized to this end by a variety of methods. The relative hydrophobicities of several PEG/salt ABSs composed of different molecular weights of PEG (1000, 2000, and 3400) and a variety of inorganic salts [K3PO4, K2CO3, (NH4)(2)SO4, Li2SO4, MnSO4, ZnSO4, and NaOH] were measured by the free energy of transfer of a methylene group DeltaG(CH2). These results indicate that the relative hydrophobicity of a PEG/salt ABS is a function of only the degree of phase divergence of the biphasic system as expressed by the difference in polymer concentration between the phases [delta poly(ethylene glycol) (APEG), delta ethylene oxide monomer (DeltaEO)] or the the line length (TLL). The distributions of a wide range of solutes differing in structure and functionality were also measured in PEG/salt ABSs, and the results were compared to the corresponding 1-octanol/water partition coefficients. These data were used to develop a linear free energy relationship (LFER) based on Abraham's generalized solvation equation, enabling a direct comparison to be made between the solvent properties of PEG/salt ABSs and those of traditional solvent/water systems used, for example, in the determination of log P. Similar comparisons are also enabled with the properties of certain aqueous micellar systems.