This paper reports the design of a tailor made polymeric membrane by using poly(ethylene oxide)-poly(butylene terephthalate) (PEO-PBT) multi-block copolymers. Their properties are controlled by the fraction of the PEO phase and its molecular weight. C To explain the effect of structural changes in copolymer membranes, transport properties of four gases (CO2, H-2, N-2, and CH4) are discussed. After characterization, the two best copolymers are selected in order to prepare tailor made blends by adding poly(ethylene glycol) (PEG). The best selected copolymer that contained 55 wt. % of 4000 g mol(-1) PEO produced a blend with high CO2 permeability (similar to 190 barrer), which is twice the permeability of the pure copolymer. At the same time, an enhancement Of CO2/H-2 selectivity is observed (similar to 13). These results suggest that the morphology of PEO-PBT can be well controlled by the addition of low-molecular-weight PEG, and consequently the gas transport properties can be tuned.