This study focuses on improving the CO2/N-2 separation performance of Matrimid thorough homogeneous blending with Rhodeftal, a new commercial high performance polymer recently introduced into the market. The membrane blends were characterized using differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) to investigate the miscibility and inter-molecular interactions between the two polymers. The DSC results showed a single glass transition temperature (T-g) between those of the individual polymers, which is a confirmation that Matrimid and Rhodeftal are miscible at molecular level. The FTIR spectra indicated strong inter-molecular interactions in this blend system. A lab-made gas permeation setup was used to evaluate the separation performance of membrane blends at different pressures. Similarly, fractional free volume (FFV) of the membrane blends was measured in order to explain gas separation results. Moreover, the plasticization behavior of the membrane blends was analyzed by pure CO2 measurements at different pressures. The results demonstrate that the blend system has significantly improved plasticization resistance to pressures up to 20 bar. In addition, when the Rhodeftal content increases, the selectivity significantly increases, whereas the permeability slightly decreases.