Developed a rapid, two-step Fenton–Thiol-Ene crosslinking strategy to modify PBI membranes. Achieved excellent acid and solvent stability, retaining >97% mass in DMAc, DMF, and NMP at 80 °C. Demonstrated high OSN performance with >95% Rose Bengal rejection and 2.1 LMH/bar ethanol flux. Employed green solvents and a 1-hour process for sustainable, scalable membrane fabrication.