Chemically modified silica are mixed with bromine modified (2,4-dimethyl-1,6-phenylene oxide or PPO), or BPPO to form nanocomposite membranes. Both modified and unmodified silica addition into the BPPO can enhance CO2 permeability without losing CO2/N2 selectivity. We find that chemically modified silica disperse much more uniformly in the BPPO polymer matrix than unmodified silica. However, all chemically modified silica-BPPO nanocomposite membranes show a much lower CO2 permeability in relative to unmodified silica-BPPO nanocomposite membranes. The mechanism of nanoparticle enhancement on gas transport in nanocomposite membranes will be discussed.