A dry/wet phase separation process has been applied to a dry/wet spinning process to prepare asymmetric polysulfone hollow fibers for gas separations. A spinning solution consisting of tetrahydrofuran as a volatile solvent, dimethylacetamide as a less volatile solvent, and ethanol as a nonsolvent was used to induce phase separation in nascent hollow fiber membranes prior to their coagulation in water. Statistical experimental designs were used to optimize the permeation properties of these fibers which showed essentially defect-free selective skin layers with thicknesses of approximately 1,200 angstrom (alpha(O2/N2) = 5.8, (P/L)(O2) = 8.8 GPU, o.d. = 580 mum). Scanning electron photomicrographs revealed many noncircular distortions in the hollow fiber lumens which appeared more prevalent at low draw ratios and when higher "water-activity" bore fluids were used. The influence of water activity in the bore fluid was investigated in detail with both salt/water and solvent/water mixtures.