Of thermosetting polymers, polyphenylene oxide (PPO) is considered as one of the alternative promising polymeric precursors for carbon-based membranes. The post-oxidized PPO derived carbon membranes were prepared by carbonization, followed by air-oxidation as post-treatment method in this study. It was observed that air-oxidation increased the pore characteristic values (pore volume, pore size and its distribution) of carbon materials. It was shown that the post-oxidation increased gas permeance due to change in the pore proper-ties. The permeation results for the post-oxidized carbon membranes showed that the extent of the permeation modification is strongly dependent on the oxidation temperature. Also, it was observed that the post-oxidation at higher temperature changed permeation mechanism from the molecular sieving to Knudsen-like diffusion especially for larger gas species. (c) 2006 Elsevier B.V. All rights reserved.