The gas permeation properties of the microporous glassy ladder polymer PIM-1 are reported for hydrocarbon/hydrogen mixtures. PIM-1 is more permeable to hydrocarbon vapors than to hydrogen, a behavior similar to that of the microporous linear-chain poly(1-trimethylsilyl-1-propyne) [PTMSP]. For a 2 vol.% n-butane/98 vol.% hydrogen feed mixture, PIM-1 has a mixed-gas n-butane permeability of 4000 Barrer combined with an n-butane/hydrogen selectivity of 27. An increase in n-butane feed concentration leads to an increase in n-butane permeability, but has only a minor effect on the hydrogen permeability. As a result, the n-butane/hydrogen selectivity increases from 27 to 51 as the feed n-butane concentration increases from 2 to 7.3 vol% Because of its excellent mixed-gas hydrocarbon/hydrogen selectivity and high permeability, PIM-1 could find use as a novel membrane material in petrochemical applications, such as hydrogen recovery from fluid catalytic cracker off-gases. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.