Polymeric membranes are convenient materials for gas separation processes such as natural gas sweetening, hydrogen purification, and air separation because they offer a simple, easy-to-operate, low-maintenance process option. However, their further development has been constrained by a performance trade-off between gas permeability and selectivity. Researchers fabricated new products by blending glassy and rubbery polymers and examined their gas separation efficiency. Here, the most recently developed polymer blends and gas transport through them have been reviewed and the mutual effects of gas transport phenomena and polymer blend composition on each other have been evaluated. It has been concluded that gas permeability increases and gas selectivity decreases as the concentration of more permeable polymer in the blend increases. The gas selectivity increases with chemical cross-linking and with decreasing temperature. The gas selectivity also decreases with pressure where CO2 plasticization occurs. © 2018 Elsevier Inc. All rights reserved.