The commercial polyimide Matrimid was modified by bromination for the purpose of improving its membrane-transport properties as a gas-separation material and providing functional group reactivity for further modifications. The unmodified and brominated. polymers were characterized by elemental analysis and one-dimensional and two-dimensional NMR, which revealed that one bromine atom per repeat unit was incorporated regioselectively onto the indane aromatic ring. The thermal and physical properties of the polymers before and after bromination were also investigated. The gas-transport properties of the unmodified and brominated. polymers were compared.