Hollow fibers were spun from a solution of surface-modifying macromolecule blended polyethersulfone in dimethyl acetamide by using dry-wet spinning method at different air gaps and at room temperature. The air gap was varied from 10 to 90 cm. The ultrafiltration performance of hollow fibers was studied by using aqueous solutions of polyethylene glycols and polyethylene oxides of different molecular weights. Significant difference in surface morphology between the inner and outer surface of the hollow fibers was observed by atomic force microscopy (AFM). Similar results were obtained by contact angle measurement and XPS. Mean pore sizes of the inner surface and outer surface were calculated from AFM images and compared with the pore sizes obtained from mass transport data. Pore size distribution curves were drawn from both data, i.e., from AFM images and mass-transport data, both methods gave similar results. Roughness parameters of the inner and outer surfaces and the sizes of nodular aggregates on both surfaces were measured. An attempt was made to correlate the above parameters with the performance of the membranes. Unexpected values of contact angles of both inner surface and outer surface were obtained. It was observed that the studied membranes could be put into two groups: (i) the membranes fabricated between 10 and 50 cm air gap and (ii) fabricated at higher than 50 cm air gap. A plausible mechanism for the unexpected results was discussed. (c) 2007 Wiley Periodicals, Inc.