Gas permeation tests using nitrogen, oxygen, hydrogen, helium and carbon dioxide were performed to assess how membrane modification procedures affect the separating layer morphology of thin-film composite reverse osmosis membranes. Gas selectivity data provided evidence for the presence of nanoscale separating layer defects in dry samples of six commercial membrane types. These defects were eliminated when the membrane surface was coated with a polyether-polyamide block copolymer (PEBAX 1657), as indicated by a 25-fold decrease in gas permeance and at least a 2-fold increase in most selectivity values. Treatment with n-butanol followed by drying reduced water flux and gas flux by 30% and 75%, respectively, suggesting that using n-butanol as a solvent for applying coatings negatively affects membrane performance. The results of this study demonstrate that gas permeation measurements can be used to detect morphological features that impact gas and water membrane flux. (C) 2008 Elsevier B.V. All rights reserved.