In this work, we proposed a novel aqueous biphasic system (ABS) composed of polypropylene glycol P400 (PPG P400) and hydrophilic ionic liquids (IL), 1-alkyl-3-methylimidazolium bromide (alkyl = ethyl or butyl), forming an upper polymer-rich phase and a lower IL-rich phase at ambient temperature. This new ABS can present interesting characteristics shared by ILs and polymers such as low volatility, good solvation ability, tunable physical properties, and high design capacity for achieving task-specific phase components to enhance the partitioning of target species. Ternary phase diagram of the novel ABS formed by PPG 400 and [C2mim]Br in water was measured at T = 298.15 K. Factors affecting the binodal curves such as the cation side alkyl chain length and the temperature were also evaluated. The results were successfully interpreted in terms of the kosmotropic/chaotropic nature of ILs. Furthermore, the phase behavior of the PPG-[C2mim]Br ABS is described by the NRTL model. Finally, the extraction potential of the proposed ABS was evaluated through its application to the extraction of the essential amino acids such as L-tryptophan and L-tyrosine. The partition coefficients here obtained demonstrated the fine potential of the proposed ABS for biomolecules separation. (c) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2012