In this work we discuss the gas transport properties behavior of dense blend membranes of a high free volume polyimide PI DPPD-IMM with PBI using the latter polymer in an attempt to enhance gas selectivity. Thermal and mechanical properties of dense blend membranes were also evaluated. The aim is to test the use of PBI as a gate polymer for establishing a window to enhance selectivity of CO₂/CH₄ and O₂/N₂ gas pairs with a minimum loss of gas permeability. Wide-angle X-ray diffraction measurements show that PI DPPD-IMM has an open structure with a d-spacing maximum at 5.4 Å, which is not affected when PBI concentration in the blends is lower than 25 wt %. This reflects a moderate loss of gas permeability as compared to PI DPPD-IMM. Blends with PBI concentrations 50 wt % or above presented a shift in WAXD toward 4.4 Å. It is found that as PBI concentration increases in the blend, there is an enhanced selectivity for O₂/N₂ and CO₂/CH₄ gas pairs duplicating their value with respect to PI DPPD-IMM and following closely the slope of the Robeson plot. Activation energies for permeation confirm that the PI DPPD-IMM microstructure is preserved at low PBI concentration since they barely increase for blends containing <25 wt % PBI. Gas permeability coefficients were fitted to different prediction models whose analysis confirms that the blends are heterogeneous. The best relationship between gas permeability and gas pair selectivity is located in the region where the blend presents high fractional free volume (FFV) (PBI concentration ≤25 wt %).