IR spectra of brominated high molecular weight PPO membranes revealed that brominating PPO at the phenyl ring results in decreasing both the C-O-C torsional motion, and the plane C-H bending, which are assigned respectively to the 1300 cm(-1) and 1180 cm(-1) bands in PPO. Splitting in the 1180 cm(-1) band was observed at 37.4% bromination and higher, indicating different modes for C-H bending that occurred as part of the methyl groups was hindered by adjacent bromine substituents. The permeability of gases in the brominated PPO membranes increased slightly in comparison to that of PPO as the degree of bromination was increased to 37.4%, however, the permeability of gases almost doubled at 60% bromination degree. The trend in the gas permeability data and the IR spectra obtained in this study both conform with and support the mechanism proposed in the literature that higher degrees of bromination are needed to enhance the permeability of gases through stiffening the PPO backbone, which increases the rate of diffusional jumps.