Biofuel has emerged as one of the most strategically important sustainable fuel sources. The success of biofuel development is not only dependent on the advances in genetic transformation of biomass into biofuel, but also on the breakthroughs in separation of biofuel from biomass. The "separation" alone currently accounts for 60-80% of the biofuel production cost. Ceramic membranes made of sophisticated processes have shown separation performance far superior to polymeric membranes, but suffers fragility and high fabrication cost. We report the discovery of novel molecular engineering and membrane fabrication that can synergistically produce polymeric membranes exhibiting separation performance approaching ceramic membranes. The newly discovered Polysulfone/Matrimid composite membranes are fabricated by dual-layer coextrusion technology in just one step through phase inversion. An ultrathin dense-selective layer made of an interpenetration network of the two materials with a targeted and stable interstitial space is formed at the interface of two layers for biofuel separation. The combined molecular engineering and membrane fabrication approach may revolutionize future membrane research and development for purification and separation in energy, environment, and pharmaceuticals. (C) 2008 American Institute of Chemical Engineers AIChE J, 55: 75-86, 2009