Composite membranes have been prepared consisting of a poly(vinyl chloride) (PVC) top layer on either a dense polyacrylonitrile (PAN) layer (bi-layer membrane) or a porous PAN support layer (normal composite membrane) and studied with respect to the dehydration of acetic acid. Especially, the influence of the surface porosity of the porous support layer on the selectivity and flux was studied and it was shown that the lower the surface porosity the higher the selectivity of the composite membrane, especially at high acetic acid concentrations in the feed. From the results it can be concluded that the support material does contribute to the selectivity. Despite the low surface porosity relatively high fluxes could be obtained. Using a feed composition of 80/20 wt.% acetic acid/water at 80°C selectivities of 182¿274 and fluxes of 0.56¿0.74 kg/m2-hr were obtained, and with a feed composition of 98/2 acetic acid/water selectivities of 206¿318 and fluxes of 0.14¿0.15 kg/m2-hr were obtained.