Blend films prepared from styrene-butadiene-styrene triblock copolymer (SBS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) in various compositions were examined to explore their application as gas separation membranes. PPO formed a single-phase mixture with hard styrene segments in SBS regardless of the blend composition. Therefore, an increase in the hard-phase volume with PPO blending resulted in phase structure changes: As the PPO content in the blend increased, the hard phase that was dispersed in the matrix formed by butadiene soft segments was converted into a continuous lamellar structure and finally formed a matrix of the blend by phase inversion. A decrease in the gas permeability and an increase in the gas selectivity were observed with increasing PPO content. The continuous soft phase was changed to the discrete phase in blends containing between 40 and 50 wt % PPO. As a result, an abrupt decrease in the gas permeability was observed at around these blend compositions. Permeability coefficients showed a small decrease as the upstream gas pressure increased when the hard phase formed a matrix, whereas they were not changed regardless of the upstream gas pressure when the soft segments formed the continuous phase. Plasticization caused by a polar penetrant was not observed in the SBS/PPO blend membranes. Our results show that blend films exhibiting desired gas permselectivity can be fabricated by controlling the phase-separated structure of SBS/PPO blends.