Free-standing graphene oxide (GO) thin films have been assembled by a pressurized ultrafiltration method. The GO films exhibit a layered microstructure with high structural stability and hydrophilicity, as analyzed by XPS, FESEM, XRD, positron annihilation spectroscopy (PAS), etc. The effects of ultrafiltration pressure and feed composition on the film's microstructure and pervaporation performance for dehydration of ethanol were investigated and correlated with XRD and PAS observations. Experimental results suggest that the interlaminar spacing is determined by both packing density of GO nanosheets and water content in the feed solution. The packing density is sensitively affected by the ultrafiltration pressure applied during film formation. By tuning the ultrafiltration pressure, a high separation performance with water permeability of 13,800 Barrer (1 Barrer = 3.348 x 10(-19) kmol m m(-2) S Pa-1) and water/ethanol selectivity of 227 is achieved for dehydration of an 85 wt% ethanol aqueous solution at 24 degrees C. Additionally, the total permeability varied from 50 to 113,000 Barrer by increasing the water content in the feed from 0 to 100 wt%. As a result, the ideal water/ethanol selectivity calculated from single-component feed tests is about 2260 which is much higher than the selectivity obtained from binary-component feed tests. This discrepancy is probably attributed to the effect of intermolecular hydrogen bonding between water molecules and the functional groups on GO nanosheets that enlarges the interlaminar spacing and allows more ethanol transport through the GO film. The interactions among the GO nanosheets and the feed components enable the film to effectively dehydrate ethanol via pervaporation. (C) 2014 Elsevier B.V. All rights reserved.