Polybenzimidazole (PBI) is highly selective for H-2 removal from mixtures with CO2, but it exhibits low gas permeabilities. Celazole (R) PBI was blended with more permeable HAB-6FDA-CI, an ortho-functional thermally rearrangeable polyimide, to improve its gas permeabilities while attempting to maintain high selectivity. A compatibilizer, 1-methylimidazole, enhanced phase dispersion when added to the casting solutions. Films containing 20-40 wt% polyimide exhibited a matrix-droplet morphology as observed by scanning and transmission electron microscopy. Heat treatment at 400 degrees C under inert atmosphere thermally rearranged the polyimide in the blends. After heat treatment, H-2/CO2 selectivities at 35 degrees C doubled for Celazole (R) and 20-33 wt% polyimide compatibilized blends, exceeding the 2008 upper bound. Simultaneous increases in both H-2 permeability and H-2/CO2 selectivity were observed after heat treatment for the 33/67 wt% HAB-6FDA-CI/Celazole (R) compatibilized blend, which exhibited over twice the H-2 permeability of Celazole (R) and similar selectivity. Heat treatment reduced free volume and induced para-crystallinity in the PBI phase of the blends as evidenced by x-ray scattering. Thermally treated blend films were mechanically flexible and exhibited elastic moduli and tensile strengths similar to Celazole (R), despite having low elongation at break.