Carbon membranes were prepared using Kapton (R) 100HN polyimide as the precursor. Carbonisations were carried out at 873 and 1073 K under argon, helium, nitrogen flow or vacuum atmosphere. The effects of carbonisation atmosphere on the membrane structure and transport properties were studied. Weight losses during thermal degradation were obtained for different inert purging gases. The values of apparent activation energy for the pyrolysis carried out under argon, nitrogen and helium at a final temperature of 1073 K were 212.83, 175.72 and 117.51 kJ/mol, respectively. Carbonisation at 873 and 1073 K under each atmosphere decreased the d-spacing and the intensity ratio of the Raman D-peak to the G-peak for the resulting carbon membranes. At these two temperatures, helium pyrolysis produced the highest BET pore surface area, total pore volume and micropore volume. Carbonisation under vacuum atmosphere at 873 and 1073 K yielded the lowest gas permeances but the highest ideal selectivities of 9.37 and 17.76 for O-2/N-2, respectively, whilst carbonisation under argon atmosphere produced the highest selectivities of 93.35 and 476.74 for CO2/CH4 for 873 and 1073 K, respectively. (c) 2007 Elsevier B.V. All rights reserved.