Membranes that permeate hydrogen and reject CO2 at temperatures above 150 °C are of great interest for low cost CO2 capture in pre-combustion processes. The current leading polymeric material for this application is poly[2,2’-(m-phenylene)-5,5’-bisbenzimidazole] (PBI), which exhibits good thermal stability, strong size-sieving ability and good H2/CO2 selectivity. In this study, we demonstrate that H2/CO2 selectivity in PBI can be significantly enhanced by doping with phosphoric acid (H3PO4). H3PO4 can bond strongly with imidazole rings in PBI, and the complexes are thermally stable up to 200 °C under vacuum. We prepares a series of H3PO4 doped PBI samples by immersing PBI thin films (12 µm) in solutions containing H3PO4 and methanol. The H3PO4 concentration can be varied from 0.05 wt.% to 1.0 wt.% to achieve different doping levels in the PBI films (0.3 - 1.0, defined as the molar ratio of H3PO4 to PBI repeating unit). Increasing the doping level increases H2/CO2 selectivity and decreases H2 permeability. For example, pure PBI exhibits pure-gas H2 permeability of 27 Barrers and H2/CO2 selectivity of 14 at 150 °C, while the PBI with a doping level of 0.44 exhibits H2 permeability of 6.1 Barrers and H2/CO2 selectivity of 59 at 150 °C. As the doping level increases to 1.0, the PBI shows an impressive H2/CO2 selectivity of 136, though the H2 permeability decreases to 1.3 Barrers at 150 °C. This performance is above the upper bound in the Robeson’s plot for H2/CO2 separation. This presentation will also discuss the effect of H3PO4 doping on structural changes such as free volume, as well as CO2 sorption and diffusion, and the structure/property relationships in these H3PO4 doped PBI thin films.