Polyethersulfone (PESf) asymmetric hollow fiber membranes were prepared by the dry/wet and the wet-phase inversion processes from spinning solutions containing N-methyl-2-pyrrolidone (NMP) and an alcohol as a nonsolvent-additive (NSA). Two grades of polyethersulfones, Victrex 4800P and Radel A-300, were used. The alcohols used in this study include five aliphatic alcohols (methanol, ethanol, l-propanol, l-butanol, l-pentanol) and two polyhydric alcohols (ethylene glycol and diethylene glycol). Water was used as both the internal and external coagulant in preparing these hollow fiber membranes. All spinning solutions were formulated such that their compositions were close to the point of incipient phase separation. The effects of the alcohols as NSA on the spinning solution properties; permeation fluxes of pure He, CO2, O-2 and N-2; and structures of the PESf hollow fiber membranes were investigated. The results show that different alcohols have a dramatically different influence on the polymer solution properties, gas separation characteristics and structures of the resulting membranes. Addition of a suitable alcohol (C-2-C-4 aliphatic alcohols) in the spinning solution greatly improved both permeability and ideal selectivity of the gases examined. The PESf hollow fiber membranes prepared from NMP/EtOH solvent systems exhibited the best gas separation characteristics comparable, if not better than those reported for other types of hollow fibers. Higher molecular weight alcohols, C, and above aliphatic alcohols as well as polyhydric alcohols were found to be not suitable in making PESf membranes for gas separation using water as coagulant.