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 ionic liquids (ILs) and concluded that transport of the compounds was mainly regulated by the affinity of the ionic liquid towards each solute. In this work, further studies were conducted in order to elucidate the molecular mechanism involved in the transport phenomena. With this aim, the permeability of 16 different organic compounds (vinyl esters, aliphatic esters, alcohols and carboxylic acids) through SLM based on ionic liquids ([bmim(+)][PF6-] and [omim(+)][PF6-]) and the partition coefficients of the compounds between each ionic liquid and the feed/receiving phase were evaluated. A clear relationship between the permeability of the compounds and the partition coefficient of the compounds between the ionic liquid and the feed/receiving phase was established. This relationship allows the easy prediction of the capability of a given SLM based on IL to separate two compounds by simply evaluating the partition coefficient of each compound between the ionic liquid and the feed/receiving phase and the molar volume of each compound at its boiling point. (C) 2007 Elsevier B.V All rights reserved.