The morphologies of polymeric integrally skinned asymmetric gas-separation membranes are typically visualized as a thin gas-selective skin region supported by a nonselective, low-resistance porous structure. The validity of this visualization for defect-free and defective membranes was investigated via scanning electron microscopy (SEM) in combination with traditional gas-permeation measurements for previously reported ultrathin defect-free hollow-fiber membranes. Hollow-fiber membranes were formed via a dry-jet, wet-quench process with a spinning solution composed of Matrimid(R) polyimide and components of varying volatility. For all the defect-free membranes formed, SEM images revealed a prominent dense skin layer across the fibers' entire outer circumference. Skin-thickness estimates from 36 SEM images and N-2, O-2, and He pure gas-permeation measurements agreed to within 9, 7, and 24% for defect-free membranes with 150-, 300-, and 600-nm skin thicknesses, respectively. Defective membranes with O-2/N-2 gas selectivities of Knudsen values, nominally 50 and 80% of dense film values, exhibited a well-developed nodular, partially developed nodular, and uniform dense layer (no nodules) skin morphologies, respectively. These results provide in-depth experimental evidence for the existence of a distinct dense integral-skin layer and suggest that the presence of nodules in the membrane's final skin morphology correlate to gas-permeation defects. (C) 2003 Wiley Periodicals, Inc.