Asymmetric blend polyethersulfone-polyimide (PES-PI) hollow fiber membranes prepared at different air gap and used for gas separation are characterized by atomic force microscopy (inside and out side surfaces) and by measuring the contact angle of out side surface. The outer surface was entirely different than the inner surface, as expected. On the inner surface nodule aggregates were aligned in rows, may be towards the direction of the bore fluid flow. On the outer surface, alignment of nodular aggregates in one direction was not observed. The average mean roughness parameter of the inner surface increased with the increase in the air gap used for the preparation of hollow fiber. On the other hand, it was opposite for the outer surface. From the gas separation experiments it was observed that gas permeation rate increased with the increase in air-gap. An attempt was made to find relationships between the surface morphology observed by AFM and permeability and selectivity of the studied hollow fiber membranes. From the contact angle measurement it was observed that the contact angle is directly related to the outside surface roughness. A plausible mechanism has been discussed. (C) 2003 Elsevier B.V. All rights reserved.