The carbon molecular sieving membranes were prepared from the polymer blend of polyphenylene oxide (PPO) and polypyrrolidone (PVP) as thermally stable and labile polymer, respectively. The permeation results for the carbon membranes derived from the polymer blends of PPO/PVP showed that the transport of gas species was affected by the molecular sieving effect and that the permeation performances had a strong dependency upon the pyrolysis temperature and PVP molecular weight. The PPO/PVP derived carbon membranes with lower PVP molecular weight than 40 K showed decreased gas permeances and increased permselectivities due to decrease in the pore structure. Meanwhile, it was observed that gas permeance for the carbon membranes of higher molecular weights than 40 K increased due to the enhanced diffusional pathways in the thermally labile polymer region. It is considered that the introduction of the thermally labile polymer leads to control the pore structure through the permeation results for the carbon membrane derived frorn the PPO/PVP and that the permeation performance is affected by the molecular weight and pyrolysis temperature. (C) 2007 Elsevier B.V. All rights reserved.