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This paper describes the preparation, characterization and the permeation properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) dense polymer films containing aliphatic hyperbranched polyesters, Boltorn (H20, H30, and H40). The Boltorn are dispersed in PPO at various concentrations. The gas permeability results were very different between low and high concentration of Boltorn. The gas permeability of PPO with 1.0 wt % of Boltorn is 2−3 times higher than the pure polymer, while at higher concentration (9.1 wt %) of Boltorn the permeability becomes almost 50% of the pure polymer. The gas pair selectivity, however, stays constant. The increase in permeability at low concentration of Boltorn is due to the increase of the free volume, probably due to hydrogen bonds between Boltorn and the oxygen of PPO backbone. The decreased permeability of PPO containing higher concentration of Boltorn (9.1 wt %) is due to two reasons:  decrease in free volume as determined by PALS as well as phase separation. The hyperbranced polyesters form aggregates that migrate to the top surface of the membrane. [ABSTRACT FROM AUTHOR]