Ionic liquids are a new class of environmentally friendly solvents which are becoming increasingly important in separation processes because they can be tailored for specific applications by an appropriate choice of their cationic and anionic components. In this work, a wide range of ionic liquids with different cation and anion compositions have been tested as liquid phase in supported liquid membranes for selectively separating the substrates and products of a transesterification reaction, with the aim of better understanding the design rules for producing ionic liquids for this particular application. It was found that selectivity values are mostly dependent on the anion composition of the ionic liquid, the highest value being reached with ionic liquids based on the methylsulfate anion. As regards cation composition, it was observed that higher selectivities were reached with ionic liquids which contain cations with shorter alkyl chain lengths. Furthermore, a relationship was established between the permeability of the compounds and their hydrophilicity which suggests that the capability of a given supported ionic liquid to separate the target compounds can be assessed simply by evaluating the Hildebrand solubility parameter of each compound. (c) 2008 Elsevier B.V. All rights reserved.