Carbonized membranes were prepared by pyrolyzing an asymmetric hollow fiber membrane of a copolyimide from equimolar portion of 2,2-bis(3,4-dicarboxyphenyl)hexafluoro-propane dianhydride (6FDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) with 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT) at temperatures of 773-973 K under a nitrogen stream. The carbonized membranes had an asymmetric structure with a skin layer of around 200 nm in thickness and a much denser support layer compared with the precursor hollow fiber. The increase in permeance to propylene R-C3H6 by the pyrolysis was up to 10 times. An appropriate holding time during the pyrolysis in a nitrogen stream tended to increase the olefin/paraffin selectivity. The membranes pyrolyzed at 813 K for 1 h displayed the best performance of R-C3H6 = 26 GPU and alpha(C3H6)/(C3H8) = 22 for mixed-component (50/50%) at 373 K and 1 atm. The effect of silicone rubber coating was also investigated. (C) 2003 Elsevier Science B.V. All rights reserved.