Brominated poly(2,6-diphenyl-1,4-phenylene oxide) (BPPOdp) is synthesized as a new membrane material for gas separation. BPPOdp forms flexible membranes with higher CO2 permeability (P-CO2 = 78 Barrer) and selectivity (alpha(CO2/N2) = 30) than poly(2,6-dimethyl-1,4-phenylene oxide) (PPOdm) membranes. BPPOdp also forms nanocomposite membranes with silica nanoparticles. The nanocomposite membranes have a greatly enhanced CO2 permeability while maintaining the CO2/N-2 and CO2/CH4 selectivities of the pure BPPOdp membranes. The CO2 permeability increases as the silica content increases in the membrane. The 10-nm silica nanoparticles are more effective at increasing the CO2 permeability than the 30-nm silica nanoparticles. Mechanism studies show that the enhanced permeability results from the nanogaps between the silica nanoparticles and the polymer chains due to their incompatibility.