The separation of H-2/CO2 is technologically important to produce the next generation fuel source, hydrogen, from synthesis gas. However, the separation efficiency achieved by polymeric membranes is usually very low because of both unfavourable diffusivity selectivity and solubility selectivity between H-2 and CO2. A series of novel diamino-modified polyimides has been discovered to enhance the separation capability of polyimide membranes especially for H-2 and CO2 separation. Both pure gas and mixed gas tests have been conducted. The ideal H-2/CO2 selectivity in pure gas tests is 101, which is far superior to other polymeric membranes and is well above the Robeson's upper-bound curve. Mixed gas tests show an ideal selectivity of 42 for the propane-1,3-diamine-modified polyimide. The lower selectivity is a result of the soprtion competition between H-2 and the highly condensable CO2 molecules. However, both pure gas and mixed gas data are better than other polymeric membranes and above the Robeson's upper-bound curve. It is evident that the proposed modification methods can alter the physicochemical structure of polyimide membranes with superior separation performance for H-2 and CO2 separation.