Aqueous biphasic systems (ABS) are relevant for the development of environmentally friendly and "bio-compatible" separation processes. However, the common polyethylene glycol (PEG) polymers present a limited range of applicability, due to the low polarity of the PEG-rich phase. To overcome this limitation, a new approach was recently proposed based on the use of ionic liquids (ILs) as adjuvants in ABS, enlarging the polarity range of these systems. This work addresses the use of imidazolium-based ILs as ionic adjuvant compounds in the formation of ABS, namely potassium salts + water + PEG (1500, 4000, 6000 and 8000). To explore the differences induced by the presence of the IL as adjuvant the partition behavior of two dyes, Chloranilic Acid and Rhodamine 6G, is analyzed and correlated with the phase behavior and the IL distribution on the ABS under study. (C) 2014 Elsevier B.V. All rights reserved.