Propylene/propane mixture separation properties of high quality carbon molecular sieve (CMS) membranes prepared on mesoporous gamma-alumina supports were studied at different feed pressures (up to 100 psia), feed compositions, and permeation temperatures. The membrane exhibits a mixture propylene/propane selectivity of above 30 under the studied conditions, with no plasticization effect observed at feed pressures up to 100 psia (similar to 700 kPa). Temperature dependences of the permeance and selectivity suggest a diffusion dominated mechanism for the permeation and separation of propylene/propane through the CMS membrane. The propylene/propane separation characteristics tend to stabilize after a decline in gas permeance and an increase in propylene/propane selectivity while on stream of propylene/propane mixture for 10 days. The initial changes in separation properties of the CMS membrane are due to the reduction in the effective pore size caused by oxygen chemisorption and physical aging of the pore structure. CMS membranes can be preaged under proper conditions to stabilize the structure and separation performance for practical separation application.