AbstractPolyethylene glycol improved the gas permeability and selectivity of Pebax® (poly(amide-b-ethylene oxide) copolymer) membrane. To understand this, the free volume variation in these blend membranes was studied from T = −80 to T = 100 °C by positron annihilation lifetime spectroscopy (PALS). A small difference in o-positronium lifetime between pristine polymer and blend with 50 wt.% of polyethylene glycol was observed between −10 and 10 °C. However, at room temperature no significant changes of free volume are detected due to the apparent molten state of polyethylene oxide. The gas permeability measurements, PALS analysis, fractional free volume (FFV) obtained from density and the theoretical correlation between diffusion coefficient and FFV show a good agreement. Thus, we could conclude that the performance of rubbery-like block copolymer membranes can also be improved by controlling the total free volume (addition of an appropriate plasticizer), and not only by solubility selectivity increase as commonly reported.