Polymeric membranes have been well established in many separation processes since the introduction of a preparation technique generating asymmetric morphologies by Loeb and Sourirajan. If the separation layer of an asymmetric membrane is highly permeable, the support layer resistance can become significant. A highly asymmetric structure over the entire cross-section of a membrane is a prerequisite for the restriction of this resistance. With regard to hollow fiber membranes, a very open pore structure on the membrane surface opposite to the active layer is required. Scanning electron microscopy (SEM) analysis of the morphology of poly(ether imide) (PEI) hollow fiber membranes prepared by a dry-wet spinning technology have been carried out. The results show that the outer layer of the extruded polymer solution can be precipitated by vapor induced phase separation (VIPs) resulting in relatively dense structures. In order to avoid VIPs, a modified air gap spinning process was developed using a triple spinneret where the outer annulus was used for transport of fluids containing a high amount of solvent. As a consequence of the presence of fluid at the outer surface of the nascent hollow fiber, the polymer concentration was locally reduced and the fiber was protected against the sorption of water vapor from the ambient atmosphere in the air gap. The results show that highly asymmetric hollow fiber morphologies without any external skin can be reproducibly prepared. Their separation properties are similar to those of conventionally prepared membranes but the permeability could be improved considerably due to a distinctly reduced support layer resistance. (c) 2005 Elsevier B.V. All rights reserved.