NOVELTY - An asymmetric modified carbon molecular sieve (CMS) hollow fiber membrane preparation process involves contacting an asymmetric polymer precursor hollow fiber polymer precursor fiber with a solution comprising a modifying agent, and contacting the obtained polymer precursor fiber with a moisture-containing atmosphere to form a treated polymer fiber. The treated polymer fiber is pyrolyzed to form an asymmetric modified CMS hollow fiber membrane having an improved gas permeance, where the concentration of the modifying agent in the solution is selected to obtain the hollow fiber membrane.    USE - Process for preparing an asymmetric modified CMS hollow fiber membrane that is utilized for separation of gases such as carbon dioxide and methane, hydrogen sulfide and methane, carbon dioxide/hydrogen sulfide and methane, carbon dioxide and nitrogen, oxygen and nitrogen, nitrogen and methane, helium and methane, hydrogen and methane, hydrogen and ethylene, ethylene and ethane, propylene and propane, and ethane/propane and ethylene/propylene.    ADVANTAGE - The process enables preparing the asymmetric modified CMS hollow fiber membrane with high strength and flexibility, and improved gas permeance, providing the ability to withstand large transmembrane driving force pressure differences, and preventing significant degradation of membrane properties by providing the first and second solvent exchange materials that are sufficiently non-reactive with the membrane.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a process for preparing a thermally rearranged polymer material having improved gas separation performance properties, which involves providing a polymer precursor material, contacting the polymer precursor material with a solution comprising a modi1zing agent, contacting the obtained polymer precursor material with a moisture-containing atmosphere to form a treated polymer precursor material, and heating the treated polymer precursor material to form a thermally rearranged polymer having an improved gas permeance.