Carbon membranes have been produced by thermo-oxidative stabilization and carbonization of polyacrylonitrile-based hollow-fibre precursors. The inner layer of coarse pores, the system of channels which penetrate through most of the wall, and the dense outer skin form the highly asymmetrical structure of the membrane. Transmission electron microscopy images of Pt/C replicas of the membrane’s outer surfaces revealed that the pore sizes and shapes in the outer skin could be altered by the use of polymers of different intrinsic viscosities as a precursor material as well as changing the stabilization atmosphere and carbonization temperature.