To combine ionic liquid (IL) recovery with cosolvent separation in biomass processing, the quaternary aqueous biphasic systems (ABSs) formed by IL, kosmotropic salt and water in the presence of dimethyl sulfoxide (DMSO) are herein proposed. Three main parameters were evaluated through the phase diagrams and the DMSO partitioning process: the IL cation and anion structure, the salt anion, and the temperature. In all systems and conditions texted, DMSO preferentially dissolves in the IL-rich phase. The results obtained indicate that, the partitioning process is essentially controlled by the IL cation interactions with DMSO. The partition coefficients (K-DMSO) displays a maximum for the system formed by [C(6)mim]Cl. The K-DMSO value increases monotonically with the initial concentration of DMSO, and it decreases in the systems with different salts: K3PO4 > K2HPO4 > K2CO3 > KOH. The increase of temperature reduces the partition coefficients. Moreover, densities and pH of both aqueous phases were measured at 298.15 K to evaluate the properties of the systems. The results gathered indicate that the densities and pH values of the two phases can be affected by the nature of IL and its initial concentrations, and the smaller density differences between the fluids, the lower the partition coefficients of DMSO. (C) 2014 Elsevier B.V. All rights reserved.