In this work, the preparation, the characterization, and the permeation properties of dense flat sheet and asymmetric hollow fiber membranes, based on BTDA-TDI/MDI co-polyimide (P84), are reported. Results are shown for pure gases and for the separation of a CO2/N2 (80/20) mixture. Dope viscosity measurements were performed to locate the polymer concentrations where significant chain entanglement occurs. Asymmetric hollow fibers were spun, using the dry/wet phase inversion process. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and the structure of the developed fibers. The permeation rates of He, CO2, O2, and N2 were measured by the variable pressure method at different feed pressures and temperatures. P84 co-polyimide proved to be one of the most selective glassy polymers. The achieved ideal selectivity coefficients are: 285–300 for He/N2, 45–50 for CO2/N2, and 8.3–10 for O2/N2, which are in the range of the highest values reported ever for polymeric membranes. The permeability of CO2 is relatively low (1 barrer, 25 °C), however it is independent of feed pressure indicating that the P84 dense membranes are not plasticized at CO2 feed pressures up to 30 bar. To the contrary, the permeance of CO2 through the asymmetric hollow fiber membranes increases with pressure, indicating that the plasticization behavior of asymmetric membranes differs from the respective dense ones. However, no evidence of plasticization was observed when a CO2/N2 (80/20) mixture was fed to the hollow fiber membranes for pressures up to 30 bar. In all cases, CO2 permeance decreased with pressure while that of N2 remained constant.