Ionic liquids (ILs) are a novel class of chemical compounds with interesting properties that are driving a lot of research in several fields. For ionic liquids to be effectively used as solvents in liquid-liquid extractions, the knowledge of the mutual solubilities between ILs and the second liquid phase is fundamental. Furthermore, while ILs cannot volatilise and lead to air pollution, even the most hydrophobic ones present some miscibility with water posturing secondary aquatic environmental risks. Despite the importance of the ILs and water systems, there are few extensive reports on their phase equilibria. The aim of this work is to discuss and understand the mutual solubilities of water and imidazolium-based ILs. The results indicate that these mutual solubilities are primarily defined by the anion followed by the cation alkyl side chain length. It was also found that the solubility of the studied ILs in water is more dependent on the ILs structural modifications than the solubility of water in those ILs. The substitution of the most acidic hydrogen in the imidazolium cation by a methyl group leads to different behaviours in both rich-phases, where the solubility of water in ILs showed to be more hydrogen bonding dependent. From the solubility results it can be concluded that the hydrophobicity of the anions increases in the order [BF4](-) < [CH3(C2H4O)(2)SO4](-) < [C(CN)(3)](-) < [PF6](-) < [N(SO2CF3)(2)](-) while the hydrophobicity of the cations increases, as expected, with the alkyl chain length increase. (c) 2007 Elsevier B.V. All rights reserved.