This study presents sub-micro porous thin polymer membranes for discriminating hydrogen (H₂) and carbon dioxide (CO₂). The membranes are fabricated by transforming amine-linked polymer (ALP) films into benzimidazole-and-amine-linked polymer (BIALP) layers. The BIALP membranes exhibit high H₂ permeance (315 GPU) and exceptional H₂/CO₂ selectivity (120), along with high pressure (up to 11 bar) and thermal (up to 300 °C) resistance. The membranes are ultra-thin (13–30 nm) and possess narrow intrinsic and transient pores, enabling precise molecular sieving for H₂ and CO₂ separation. The membranes were synthesized via interfacial polymerization and thermal treatment, with the pore structure controlled by pH adjustments. The membranes showed excellent performance in separating H₂ and CO₂ under industrial conditions, with BIALP membranes demonstrating high H₂ permeance and selectivity even under humid conditions. The study highlights the potential of BIALP membranes for efficient gas separation applications. The membranes were characterized using various techniques, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results demonstrate that the BIALP membranes outperform conventional polymer membranes in terms of H₂/CO₂ separation performance. The study provides a new approach for designing porous polymeric membranes for precise molecular discrimination.This study presents sub-micro porous thin polymer membranes for discriminating hydrogen (H₂) and carbon dioxide (CO₂). The membranes are fabricated by transforming amine-linked polymer (ALP) films into benzimidazole-and-amine-linked polymer (BIALP) layers. The BIALP membranes exhibit high H₂ permeance (315 GPU) and exceptional H₂/CO₂ selectivity (120), along with high pressure (up to 11 bar) and thermal (up to 300 °C) resistance. The membranes are ultra-thin (13–30 nm) and possess narrow intrinsic and transient pores, enabling precise molecular sieving for H₂ and CO₂ separation. The membranes were synthesized via interfacial polymerization and thermal treatment, with the pore structure controlled by pH adjustments. The membranes showed excellent performance in separating H₂ and CO₂ under industrial conditions, with BIALP membranes demonstrating high H₂ permeance and selectivity even under humid conditions. The study highlights the potential of BIALP membranes for efficient gas separation applications. The membranes were characterized using various techniques, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results demonstrate that the BIALP membranes outperform conventional polymer membranes in terms of H₂/CO₂ separation performance. The study provides a new approach for designing porous polymeric membranes for precise molecular discrimination.