Abstract A series of new carbon molecular sieve fibers (CMSF) was made by a two-step pyrolysis of melt extruded polyvinylidene chloride copolymer (PVDC) fibers. The PVDC fibers were first pretreated in air at low temperature to dehydrochlorinate and crosslink, and then pyrolyzed in N2 at high temperatures from 500 to 1400 °C. A high throughput kinetic adsorption tool with 48 parallel testing cells using 9 probe molecules (kinetic diameter from 3.3 A −5.5 A) was used to 1) characterize the effective micropore size and its changes, and 2) fast screen the adsorbents for several gas separations. The effective micropore size continuously shrank from ~4.9 to 3.4 A, as the final pyrolysis temperature increased from 500 to 1400 °C. The CMSF adsorbents can be tuned for several gas separations: N2/CH4, C2H4/C2H6, and C3H6/C3H8. The newly discovered CMS has 54% larger capacity and 86% larger working capacity (in 13–250 kPa pressure range) of propylene than the commercial MSC-4K CMS adsorbent. By reducing the diameter of CMSF from 100 to 50 μm, the adsorption kinetics was improved while maintaining good separation factor in the range of 12–29. Furthermore, this new CMSF adsorbent has potential to form future advanced structured adsorbents for rapid cycle pressure swing adsorption to achieve a smaller adsorbent bed.