Dense flat membranes of sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and aromatic diamines of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane disulfonic acid, benzidine-2,2 '-disulfonic acid, 4,4 ' diaminodiphenylether-3,3 '-disulfonic acid and 9,9 '-bis(4-aminophenyl)fluorene. The SPI membranes were pyrolyzed at a relatively lowtemperature of 450 degrees C for 1.5 h at a nitrogen flow. During the pyrolysis, the -SO3H groups decomposed without substantial cleavage of the polyimide backbone. The pyrolytic membranes had the interesting intermixed properties of toughness and good flexibility as in polymer membranes and high gas permeability with reasonably high selectivity as in carbon molecular sieve membranes. The gas permeability for the pyrolytic membranes seems to be controlled by the sulfonic acid group content in the precursor polyimides, the relaxation of the polymer chains during the pyrolysis and the gas permeation level of the precursor membranes. Decomposition Of -SO3H groups induced microvoids, which are considered to remain to some extent as larger-size free volume holes in the polymer matrix, resulting in the higher diffusivity to gas with larger diameter. The pyrolytic membranes showed the higher permeability Of C3H6 and/or the higher selectivity Of C3H6/C3H8 than the 6FDA-polyimide membranes which have defined the upper bound line. (c) 2005 Elsevier B.V. All rights reserved.