Asymmetric hollow fibers have increasingly become attractive for high volume gas separation applications including natural gas purification. The formation of asymmetric structure with ultrathin defect-free selective layer and mechanically robust porous supporting layer without additional coatings is one of the major challenges. In this study, a commercially available polyimide material Matrimid (R) was used as the benchmark polymer material in fabrication of hollow fibers for natural gas purification. Major fabrication process variables including polymer solution formulation, bore fluid composition, dope and bore flow rates, and air-gap length were systematically investigated and optimized for fabrication of hollow fibers to achieve high CO(2)/CH(4) separation performance. SEM images of cross-section indicated asymmetric structure of hollow fiber with a skin layer and sponge-like supporting structure. Hansen's solubility parameters were calculated for different solvent systems and coagulation medium, and the predicted trends of their effect to the formation of skin layer and gas separation performance were consistent with the experimental observation. High CO(2)/CH(4) separation factors (ranging up to 67) are achieved which are among the highest reported for purely polymeric hollow fibers without post-treatments and exceeded the commonly reported bulk values of Matrimid. (C) 2010 Elsevier B.V. All rights reserved.