This article introduces a new postsynthetic modification method for tuning the separation properties of carbon molecular sieve (CMS) membranes. Polymeric hollow fibers of 6FDA/BPDA-DAM were pyrolyzed to 550 degrees C under inert Argon, and were then exposed to a tetrahydrofuran solution containing PPM levels of paraphenylenediamine dopant. The original goal of the treatment was to modify the morphology in a manner to prevent relaxation of the CMS to suppress physical aging, envisioned to be analogous to that in glassy polymer membranes where reduction of unrelaxed free volume causes reduced permeance with a mild increase in selectivity. Results of long-term CO2 and CH4 permeation experiments of doped fibers showed that aging-resistant membranes did not result. This fact notwithstanding, results revealed this approach provides a valuable new tuning tool for CMS transport properties. Complementary gas sorption experiments performed suggest the amine dopants tune large ultramicropores, thereby increasing size and shape based diffusion selectivity. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 751-760, 2017