A series of aromatic polyamide membranes for the pervaporation separation of aqueous ethanol mixtures was investigated. It was found that the permeation rate could be increased by the introduction of bulky substituted groups and arylene ether groups into the polymer backbone. The influence of the substituted group structure on the free volume in and the pervaporation performance of the aromatic polyamide membranes were systematically analyzed by positron annihilation spectroscopy (PAS) and molecular dynamics (MD) simulation. The trend of the ortho-positronium (o-Ps) lifetime and the free-volume size data evaluated by the PAS measurement and the MD simulation was highly consistent with the chemical structure of the aromatic polyamide pervaporation membranes. (C) 2009 Elsevier B.V. All rights reserved.