The effects of water flow rate (WFR) (5 or 7.5 mL/min) and length of air gap (LAG) (in the range of 50 to 120 cm) on the characteristics of hollow fiber membranes produced by the solution spinning technique using two polymer solution compositions C1 and C2, were studied experimentally. The polymer (polyethersulfone), solvent (1-methyl-2-pyrrolidone), and additive (polyvinyl pyrrolidone) concentrations (wt%) were 20, 75, and 5 respectively for the C1 solution, and 20, 70, and 10 respectively for the C2 solution. The viscosity of the C1 solution used was 2112 cP and that of the C2 solution used was 3924 cP. The extrusion pressures (EP) were 5 and 15 psig, respectively, for fiber production from the C1 and C2 solutions. The effect of higher solution viscosity, together with higher EP, was to increase both the outside diameter (OD) and the inside diameter (ID) for C2 fibers. For both C1 and C2 fibers, an increase in WFR at a given LAG tended to increase OD and ID and to decrease wall thickness, whereas an increase in LAG at a given constant WFR tended to decrease OD, ID, and wall thickness for the resulting fibers. The UF separation (%) of PEG solutes (of different molecular weights) in dilute aqueous solutions and the membrane permeated product rates (PR, g/cm(2) . h) at the average operating pressure of 20 psig were also examined as functions of WFR and LAG. Data on C1 fibers showed that for the case of WFR = 5 mL/min, an increase in LAG tended to increase both PEG separations and PR; at WFR = 7.5 mL/min, an increase in LAG tended to increase PEG separations but to decrease PR. The data on C2 fibers showed that both PEG separations and PR increased with an increase in LAG, and decreased with an increase in WFR. All the above results are discussed from the points of view of the physicochemical events of desolvation, fiber swelling, and fiber stretching taking place during fiber production.