In this study, polyethersulfone (PES) hollow fiber ultrafiltration membranes with enhanced flux have been developed for removal of humic acid. The membranes were spun front dope solutions containing PES/poly(vinyl pyrrolidone) (PVP)/N-methyl-2-pyrrolidone (NMP)/additive(s) by using the dry-jet wet-spinning process. Effects of additive such as water, LiNO3, and 1,2-propanediol (1,2-PD)in dope and air gap length were investigated. Characterization of the membranes in terms of pure water flux and apparent mention for humic acid were conducted at a steady state under almost identical hydrodynamic operating conditions. The experimental results showed that all additives in the dope dramatically enhanced the flux of resultant membranes but reduced the retention depending on the additive. For 1,2-PD as an additive, the fiber with an inner skin spun under an air gap of 600 mm had higher both flux and retention than the wet-spun fiber with an outer skin. However. for LiNO3 as an additive. 2 mm increase in air gap significantly enhanced both flux and retention. 1,2-PD enhanced the membrane flux but reduced the retention much significantly than LiNO3. The best membrane had a pure water flux of 110 x 10(-5), L m(-2) h(-1) Pat and the nominal MWCO 6000 with a retention rate of over 97% for humic acid. The SEM images revealed that the designed morphology of hollow fiber membranes could be obtained by adjusting the relative coagulation rate at each surface of the nascent fiber (adjusting composition of the bore fluid and air gap length in this study), resulting in a single skin at either outside or inside and a porous surface on the other side. (C) 2004 Elsevier B.V. All rights reserved.