In this study the effect of the composite membrane structure on the mass transfer limitations in hydrophobic pervaporation of dilute aqueous solutions of homologous series of alcohols and esters was investigated. Three hydrophobic membranes with poly (octyl)methylsiloxane (POMS) as an active layer were used. The membranes studied varied with respect to the thickness of the active layer and the nature and geometric characteristics of the support layer: polyetherimide (PEI) or polyacrylonitrile (PAN). The transport through the support was modelled by Knudsen diffusion. While the support layer had a moderate effect on mass transfer resistance for homologous series of esters, for the homologous series of alcohols the effects of the support layer depended not only on the geometric characteristics of the support structure, but also on the nature of the support: the membrane with the PEI support exhibited a greater reduction in driving force than membranes with the PAN support. However, regarding selectivity towards the feed mixture the membrane with the PEI support was superior, due to the correct proportions between the thickness of the active layer and the support layer. Further improvement in membrane performance can be obtained by optimizing the geometric characteristics of this support structure in order to minimize the reduction in driving force of pressure-sensitive compounds. (c) 2005 Elsevier B.V. All rights reserved.