Ultra-thin and defect-free polyimide hollow fiber membranes were formed using a dry-jet, wet quench process with spin dopes both with and without volatile solvents. Fibers were spun from the commercially available polyimide, Matrimid(R) 5218, and were precipitated in an aqueous quench bath. Spin dopes comprised volatile and non-volatile solvents, polymer, and non-solvent. The influences of dope composition, spinning parameters, and dehydration procedures on the membrane morphology and performance were investigated. Without post-treatment (i.e,, defect repair), the fibers exhibited skin thicknesses on the order of 1000 Angstrom and O-2/N-2 selectivities ranging from 90 to 100% of those determined for dense, solution-cast films. The 250 mu m OD/125 mu m ID fibers were spun at take-up rates comparable to those used in commercial processes (i.e., 50 m/min) and had macrovoid-free morphologies,