Dehydration of ethanol/water mixtures was performed by a pervaporation process using asymmetric hollow fiber membranes made from commercially available polymers: Matrimid (R) (polyimide) and Torlon (R) (polyamide-imide). The as-spun membranes showed low separation factors of ethanol and water mixtures. To increase their separation performance, thermal annealing of the fibers was performed. Thermally annealed fibers showed markedly increased separation performance, and this improvement depends upon the annealing conditions (both annealing temperature and time). The calculated intrinsic selectivities of the annealed Matrimid (R) fibers are 416 and 275 for 15 wt% and 95 wt% ethanol feeds, respectively. However the observed separation factors for the corresponding 15 wt% and 95 wt% feeds are 40 and 240, respectively. The decrease in the separation factors, compared to the intrinsic selectivities, especially for 15 wt% ethanol feed, is due to the unfavorable relative vapor pressures of the components and their unfavorable relative activity coefficients. The Torlon (R) fibers showed separation factors of 130 at 85 wt% ethanol feed. Long term stability tests were performed on the annealed Matrimid (R) fibers at low and high ethanol feeds. The annealed Matrimid (R) fibers showed stable separations over time at low ethanol concentrations (15 wt% ethanol), however at higher ethanol concentrations (85-95 wt% ethanol), the membranes eventually become swollen and plasticized by ethanol resulting in decreased separation performance. Crosslinking of the polymeric chains, therefore, appears to be necessary to stabilize the structures for long term application. (c) 2008 Elsevier B.V. All rights reserved.