Intercalation of alkali metals into carbon molecular sieve (CMS) membranes was studied to get new materials for gas separation. The preparation was carried out in ether solutions, containing phenanthrene and alkali metal. The mechanical stability was satisfied for potassium doped CMS, prepared in MeTHF. XRD analysis suggested a random stage structure for the modified membrane due to the low crystallinity of the original host carbon. Characterized by time-lag method some increase in the He and Nz permeability was observed by intercalation, because the graphite-like hexagonal layers were expanded and/or larger pathways were formed, so that the gas molecules could easily diffuse through. One membrane doped with potassium, followed by partial de-intercalation, exhibited an interesting perm-switch behavior at around 323 K.