This study, for the first time, demonstrates a new double-crosslinking approach to manipulate the microstructure and gas separation performance of PBI membranes for H-2/CO2 separation at 150 degrees C. The PBI membranes were firstly blended with sulfonated polyphenylsulfone (sPPSU) as an ionic-crosslinker and then alpha,alpha'-dibromo-p-xylene (DBX) as a covalent crosslinker with the aid of thermal annealing. Experiments show that PBI and sPPSU interact on the molecular level. Different from conventional blends, the PBI/sPPSU blend membranes show both enhanced chemical resistance and greater fractional free volume (FFV) after annealing at elevated temperatures because of chain motion and ionic crosslinking. As a result, the blend membranes maintain their high permeability after annealing. The DBX addition further improves the chemical resistance and tightens the inter-chain spacing that results in membranes with a small FFV and pore size. Consequently, the DBX cross-linked blend membranes retain their high gas pair selectivity at high temperatures. The 50/50 PBI/sPPSU blend membrane crosslinked by 0.7 wt% DBX at 300 degrees C possesses the best separation performance surpassing the Robeson's upper bound. It has an impressively H-2 permeability of 46.2 Barrer and a high H-2/CO2 selectivity of 9.9 at 150 degrees C.