This work evaluates the use of room temperature ionic liquids (RTILs), based upon the 1-n-alkyl-3-methylimidazolium cation, in supported liquid membranes (SLMs). RTILs are thermally stable salts, liquid at room temperature, constituted by an organic cation and either an organic or an inorganic anion. Because they are air and water stable, have a non-measurable vapour pressure and are able to solvate a variety of organic and inorganic species, ionic liquids are emerging as alternative green solvents, namely as reaction media for synthesis, catalysis and biocatalysis. The ionic liquids used in this work, 1-n-alkyl-3-methylimidazolium salts, [CnMIM](+)[X](-), possess relatively high viscosities and interfacial tensions and, depending on the anion, they may possess a reduced solubility in water. These properties make their use very attractive in order to obtain stable supported liquid membranes. This work investigates the operational stability of these supported liquid membranes and the mechanisms that regulate solute transport across them. In particular, the effect of water solubilised in the RTILs and the mechanisms involved in water transport through the corresponding SLMs were investigated. The effect of water mobility on the transport of small water-soluble ions, and on the partitioning and transport behaviour of larger water-soluble molecules was also investigated. (C) 2004 Elsevier B.V. All rights reserved.