Defect-free polymeric dual-layer hollow fiber membranes consisting of an ultrathin dense-selective polyamide imide (PAI) layer and a polyethersulfone (PES) supporting layer have been successfully fabricated in this study for gas separation application. It is observed that a lower outer-layer dope flow rate does not necessarily result in the formation of an ultrathin dense-selective layer upon the PES supporting layer. An optimization in the velocity between the inner-layer and the outer-layer dopes at the exit of the spinneret is essential to minimize additional stresses and defect formation in the outer functional layer. The best gas separation performance of the PAL-PES dual-layer hollow fibers fabricated in this study has an O(2)/N(2) selectivity of 7.73 with a dense-selective layer thickness of 63 nm. Positron annihilation spectroscopy (PAS) has been used for the first time to explore the morphology and predict the gas separation performance of PAI-PES based dual-layer hollow fiber membranes. Doppler broadening energy spectra (DBES) from PAS accurately estimate the outer-layer thickness and demonstrate the existence of the multilayered structure of the dual-layer hollow fiber membranes. Besides, the PAS fitted data reveal that the fiber spun under the optimal condition has the densest selective layer, which agrees well with the highest gas-pair selectivity observed under this condition. (C) 2011 Elsevier ay. All rights reserved.