The morphology of porous polysulfone hollow fibers which were spun by the dry–wet spinning process is discussed. It was demonstrated that a relatively moderate quenching medium should be employed in the bore of the nascent fiber in order to produce an isotropic fiber free of macrovoids and intrusion cells. A rather delicate quantitative balance between the internal precipitant and the spinning solution has to be maintained, especially when low-viscosity polymeric solutions are employed. Scanning electron micrographs of fiber cross sections display highly porous, spongestructured walls which in some instances exhibit a rather dense interface skin. However, control of the extrusion/coagulation procedure allows the formation of skinned, porous skinned, and nonskinned fibers.