This work extends earlier investigations of effects of pyrolysis atmosphere on gas separation performance of carbon molecular sieve (CMS) membranes. Based on a review of prior studies, we formulated a hypothesis that CMS performance can be controlled by tuning the amount of oxygen present during pyrolysis. In this work, a fluorinated polyimide precursor for CMS membranes was explored, using a maximum temperature of 550 degrees C and 2 h soak time at 550 degrees C. The polymer was pyrolyzed under (i) vacuum and (ii) inert pyrolysis, using a mixed gas containing specific amount of oxygen (4, 8, 30, or 50 ppm). The total amount of oxygen exposure and amount of oxygen consumed were calculated in each case, and separation performances were evaluated for each case. A strong correlation was found between the amount of oxygen available during pyrolysis and separation performances, thereby supporting the above hypothesis. (C) 2009 Elsevier B.V. All rights reserved.