Hydrophobic composite membranes for dehydrating aqueous alcohol mixtures by pervaporation were investigated. It was suited for applications at high temperatures and for feed mixtures of water and alcohols with high number of carbon atoms. With increasing operating temperatures, pervaporation data indicated simultaneous increase in permeation flux and concentration of water in permeate. At 70 degrees C and for a feed mixture of 10 wt% water and 90 wt% ethanol, the composite poly(2,6-dimethyl-1,4-phenylene oxide) membrane delivered a high flux of 1333 g/(m2h) and a high permeate content of 96.4 wt%, H2O. For a feed with higher water content, the membrane performance was further enhanced. The microstructure of the selective layer was probed with positron annihilation lifetime spectroscopy at varying temperatures. A bimodal free volume distribution was obtained, which described different shifts of free volume as temperature increased: a shift of smaller free volume to a region of much smaller free volume and a shift of bigger free volume to a region of much bigger free volume. (c) 2014 Elsevier Ltd. All rights reserved.