A novel polymer of intrinsic microporosity, thioamide-PIM-1, has been prepared by postmodification of PIM-1, using phosphorus pentasulfide as a thionating agent in the presence of sodium sulfite. The chemistry was first tested on a low molecular weight model compound, 3,13-dicyanobenzo-1,2,4',5'-bis(1,4-benzodioxane). For the polymer, up to 80% conversion of nitrile to thioamide was achieved. Modification leads to a reduction in the BET surface area of the polymer, from 770 m(2) g(-1) for the parent PIM-1 to 263 m(2) g(-1) on 80% conversion of nitrile to thioamide. After modification, the polymer is no longer soluble in chlorinated solvents such as CHCl(3), but is soluble in polar aprotic solvents such as THF, DMF, DMSO, and DMAc. Self-supported membranes of thioamide-PIM-1 were cast from THF. Single gas permeability measurements showed increased selectivities but reduced permeabilities compared to the parent polymer. Ethanol treatment, which removes occluded THF as well as enhancing free volume, has a pronounced effect on membrane permeability. For the as-prepared membrane, an ideal CO(2)/N(2) selectivity of 38.5 was achieved with a CO(2) permeability of 150 barrer. Ethanol treatment increased the CO(2) permeability to 1120 barrer, with a decrease in CO(2)/N(2) ideal selectivity to 30.3.