Abstract Asymmetric hollow fibers with the skin on the inside of the fiber were prepared from a three-component dope mixture containing polysulfone (18 wt%) polyvinylpyrrolidone (18 wt%) and N, N-dimethylacetamide (64 wt%). A wet—dry spinning technique was employed, using a tube-in-orifice spinneret. Water was used as the inside coagulation agent. The principal variables of the spinning process studied are the average velocity of the polymer solution through the spinneret, the rate of the inside coagulation agent and the air gap between the spinneret and the collecting bath. The results show that the spinning conditions have a pronounced effect on the ultrafiltration characteristics. As extrusion rate was decreased from 8 cm/sec to 1.6 cm/sec, the air gap decreased from 50 to 25 cm, the water flux inside the nascent fiber increased from 4 X 10 −2 to 6 X 10 −2 cm 3/sec, the hydraulic permeability increased from 2 X 10 −8 to 6 x 10 −8 cm/sec-Pa, while rejection for polyvinylpyrrolidone ( M w ≅ 10,000) decreased from 0.8 to 0.4. An empirical correlation is proposed which represents all results. The experimental findings are discussed in terms of the orientation introduced into the polymer solution as it passes through the spinneret.