Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) of different intrinsic viscosities has been studied to understand the effect of polymer molecular weight on the permeability and permeability ratio of CO2/CH4 and O-2/N-2 gas pairs. The increase in permeability of dense films prepared from higher molecular weight PPO was explained in terms of increased free volume. Gas permeability for the high molecular weight was further improved by attaching bulky bromine groups to the phenyl ring of the PPO backbone. Permeability ratio of PPO was greatly improved by attaching polar groups such as -COOH or -SO3H. The loss in permeability because of the presence of the polar groups was compensated by using PPO that was brominated and sulfonated.