Abstract The preparation of carbon hollow fibers by an asymmetric precursor is reported. The effect of the pyrolysis environment on the structure and properties of the produced membranes, both on inert (N 2) and reactive (CO 2 and H 2O) conditions, have been examined by a carbonization process up to 900 °C. The surface structure and morphological characteristics were investigated using scanning electron microscopy (SEM). The produced carbon fibers exhibit H 2 permeance varying from 20 to 52 GPU with a highest H 2/CH 4 permselectivity coefficient of 137 (M3) and very high H 2/CO 2 permselectivity coefficient of 37.82 (M1). Permeation rates of He, H 2, Ar, CH 4, CO 2, CO, O 2 and N 2 at variable pressure were measured too. The porosity of the developed membranes was estimated by nitrogen adsorption at 77 K. The results suggest that the formation of the selective layer on the fibers is independent from the initial orientation of the asymmetric polyimide precursor. The size of the pores is influenced by the temperature of the carbonization process whereas the pore volume by the pyrolysis environment conditions.