High-performance thin-film composite polyamide membranes for pervaporative dehydration processes were prepared by means of the interfacial polymerization of triethylenetetramine (TETA) and trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane support. The effects of annealing processes applied during and after the composite membrane preparation on the pervaporation performance were investigated. Two annealing processes were applied. One was to the composite membrane formed after the interfacial polymerization by subjecting it to different annealing temperatures and the other to the aqueous amine (TETA) solution used during the interfacial polymerization process by varying its temperature. Positron annihilation spectroscopy experiments using a slow positron beam were carried out. One of the positron annihilation techniques. Doppler broadening energy spectroscopy (DBES), was used to study the effect of the annealing processes on the S parameter (corresponding to the free volume in the membrane). The first plateau (S-1 = 0.47798 +/- 1.86E-4, L-1 =182 +/- 84 nm) in the curve obtained for the polyamide layer in the Type-B composite membrane was higher than that (S-1 = 0.47659 +/-2.74E-4, L-1 = 196 +/- 52 nm) in the Type-A composite membrane. In other words, this corresponds to a higher S parameter and a thinner layer for the Type-B membrane than the Type-A membrane. This implies that the free-volume amount in the former membrane is higher than that in the latter. It was found that these two annealing processes greatly improved the pervaporation separation performances of the thin-film composite polyamide membranes prepared in this study in dehydrating aqueous alcohol solutions. (C) 2010 Elsevier B.V. All rights reserved.