In this article, we present a facile approach, pyrolysis end-doping (PED), to finely tune pore structures to optimize separation properties of polyimide-derived asymmetric carbon molecular sieve (CMS) hollow fiber membranes. The PED approach creates CMS fibers with excellent transport properties using argon with 10 ppm oxygen for 1 min at the end of a conventional high-temperature pyrolysis process. The tuning capability is demonstrated for a membrane with a thin selective skin layer of about 1 mu m thickness for already highly permeable 6FDA-DETDA:DABE polyimide-derived CMS membranes to improve the CO2/CH4 permselectivity. In the PED process, trace oxygen reacts primarily with the skin layer and underlying support transition layer of ultramicropores and micropores to achieve overall membrane separation property improvements for natural gas purification. The PED is a simple one-step process that is easier to implement than a previously published dual temperature secondary oxygen doping (DTSOD) process. The PED approach, therefore, adds a valuable and scalable new tool, ideally suited for fine-tuning of membrane properties.