AbstractThe structures of polysulforne hollwo fiber membranes, spun from the propionic acid :N‐meth‐ylpyrrolidone complex and from a formylpiperidine/formamide mixture were investigated as a function of progressive surface removal with an oxygen plasma. Oxygen plasma ablation experiments were performed on both unexposed and isopentanetreated hollow fiber membrances. Pure gas permeation rates were obtained on these samples as well as oxygen plasma etched samples which were then subsequently coated with polydimethyl‐siloxane from an isopentane solution. The results show that the hollow fiber membrane spun from the propionic acid :N‐methylpyrrolidone complex has both a thinner active separating layer and a thinner skin than the polysulfone hollow fiber membrane spun from the formylpiperidine/formamide mixture. Also, the resistance to flow of the porous substrate of the complex spun hollow fiber membrane is significantly less than that of the polysulfone hollow fiber membrane spun from the mixture. Therefore, the substrate of the PA : NMP complex spun hollow fiber membrane has greater porosity and less tortuosity than its FP/FA congener. The oxygen plasma ablation results and the scanning electron micrographs demonstrate a nonequivalence between the active separating layer and the microscopically observable skin of the hollow fiber membrane. It is believed that membranes prepared from Lewis acid : base complex solvents possess a porous substructure and a nonuniform (graded‐density) skin which consists of a very thin active separating layer whose effective thickness varies depending upon the gases to be separated and a thin less dense transition layer, which may contain pores whose sizes are below the limits of resolution by SEM. Both are components of the microscopically observable skin. Membranes possessing this structure belong to the trilayer class of integrally skinned membranes. If membranes are so fabricated that the density gradient in the active separating layer approaches zero, a bilayer membrane with a porous substructure and a thin skin of uniform density results. Membranes prepared from conventional solvent/nonsolvent mixtures, i.e., formylpiperidine/formamide, exhibit a diminished density gradient in the skin approximating the bila