Block copolymers are under growing consideration as precursor materials for use in a wide variety of emerging nanotechnologies. While these materials can serve as ordered templates in the preparation of nanoporous membranes, they can also be designed for use as dense nanostructured polymer membranes exhibiting chemical specificity. In the present work, we explore the properties of a poly(styrene-b-ethylene oxide-b-styrene) (SEOS) triblock copolymer and its blends with poly(ethylene glycol) (PEG) as reverse-selective membranes due to their unusually high CO2 affinity. The permeability of CO2 measured as a function of blend composition, PEG molecular weight, and temperature is consistently found to exceed that of any other gas (H-2, N-2, or O-2) examined here. Addition of PEG eventually results in a composition-dependent transition from an alternating lamellar to polyether-continuous morphology, as evidenced by both gas-transport and thermal properties, and a systematic variation in crystallinity that depends on PEG molecular weight. Since the microphase-ordered copolymer morphology remains intact up to temperatures higher than the polyether melting temperature, the changes in permeability that occur upon polyether melting can be directly measured.