Previously, we reported the selective simultaneous separation of the substrates and products of a transesterification reaction (vinyl butyrate, 1-butanol, butyl butyrate, and butyric acid) through supported liquid membranes (SLMs) based on two ionic liquids (ILs): 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim(+)][PF6-], and 1-octyl-3-methylimidazolium hexafluorophosphate, [omim(+)][PF6-]. The significant differences observed in the selectivity values, attributed to the different nature of the ionic liquid phase used, led us to further investigate this matter. In this work, six ionic liquids were tested as liquid phase in SLMs for the selective separation of vinyl butyrate, I-butanol, butyl butyrate, and butyric acid. The influence of the IL as liquid phase in the resulting SLMs was analyzed measuring the values of permeability for each organic compound through the SLMs. In all cases, significant permeability differences between the organic compounds were obtained. The hydrophilic/hydrophobic character of the IL used, measured by the octanol-water partition coefficients (K-ow), was found to be a key parameter for the selective behaviour of the SLM. It was observed that a decrease in the K-ow of the ionic liquid resulted in a higher separation efficiency of the resulting SLM. (C) 2007 Elsevier B.V. All rights reserved.