Polybenzimidazole (PBI) thin films were chemically crosslinked using 1,3,5-Tris(bromomethyl) benzene (TBB) to manipulate the microstructure of polymer chains and to achieve a high sieving ability for H-2/CO2 separation. By changing the TBB concentration and TBB/PBI molar ratio in the crosslinking solutions, PBI films were modified with different degrees of crosslinking. The increment of crosslinking density in the membranes leads to a decrease in fractional free volume (FFV), indicating the tightening effect of the TBB crosslinking reaction on PBI films. The H-2/CO2 mixed gas separation was conducted at 150 degrees C. The membrane with the highest crosslinking density and the lowest FFV possesses the best performance for H-2/CO2 separation with a H-2 permeability of 9.6 Barrer and a remarkable H-2/CO2 selectivity of 24. It surpasses the Robeson's upper bound and some of other membranes, indicating its promise for hydrogen purification and CO2 capture at elevated temperatures.