Hollow fibers were spun from a polyetherimide (PEI) solution in n-methylpyrrolidone (NMP) and gamma-butyrolactone (GBL) by using the dry-wet spinning method at different air-gaps. The ultrafiltration performance of hollow fibers was studied using aqueous solutions of polyethylene glycol of different molecular weights. The pure water permeation flux increased with an increase in the air-gap. Molecular weight cut off (MWCO) also increased with an increase in the air-gap. Significant difference between the inner and outer surface of the hollow fibers was observed by atomic force microscopy (AFM). Alignment of nodule aggregates in the direction of bore fluid (water) was observed at the inner surface. The roughness parameters decreased both at the inner and outer surface with an increase in the air-gap but the changing pattern was quite different. On both surfaces, the size of nodule aggregates decreased with an increase in the air-gap. As the roughness parameter decreases, the pore size increases. A plausible explanation has been made. (C) 2004 Elsevier B.V. All rights reserved.