Hollow fibers with ultrathin skins for gas separation have been fabricated from a binary (37 wt.% PES in NMP) solution. Effects of orientation relaxation and bore fluid chemistry on morphology and performance of the membranes have been investigated. An outer skin membrane was successfully fabricated for the first time from this binary spinning dope with a very short air gap of 0.5 cm and with a bore fluid of 80 wt.% NMP in water. The experimental results showed that the permeances of all the tested gases for the unwet-spun fiber were much higher than that of the wet-spun fiber but the selectivities of the former membrane were lower than that of the latter, which indicates that the unwet-spun fiber had a much less dense outer skin than the wet-spun fiber. The results confirmed that the molecular orientation induced within the spinneret at the outer surface of the nascent fiber could relax in the very short air gap, which originally could be frozen into the wet-spun fiber. For the wet-spun fibers, the permeances of all the tested gases decreased while the selectivities increased when bore fluid rate decreased or when the concentration of solvent in the bore fluid decreased. It was concluded that the chemistry of bore fluid had a significant effect on the microstructure and performance of final fibers. The newly developed PES hollow fiber had a selectivity of 5.7 for O-2/N-2 with a permeance of 11.3 x 10(-6) cm(3) (STP)/cm(2) s cm Hg for O-2 at room temperature and the effective thickness of the membrane skin layer was 360 Angstrom from the calculation. Aging study (in air) on different fibers showed that permeances Of O-2 declined by similar to50% and selectivities of O-2/N-2 decreased by 10-15% after 130 days then leveled off. (C) 2003 Elsevier B.V. All lights reserved.