Permselective separation of aromatic from non-aromatic hydrocarbons in feed streams can be accomplished by using membrane systems in hyperfiltration mode. In petrochemical or refinery operations, feed streams containing aromatics such as benzene, toluene, or xylenes can be selectively enriched or depleted in aromatics content. The development program for the STARMEM™ series of membranes led to a commercial process for their large-scale manufacture. A range of membranes in terms of flux and selectivity has been made, and both 2-inch and commercial scale 8-inch diameter spiral wound elements manufactured from these membranes. Tests at 800 psi (55 bar) and 50 °C with a toluene recycle stream have shown stable operation of these modules in hot hydrocarbons. The high efficiencies of these modules allow for high stage cuts (50% and greater) from process streams of thousands of barrels per day. By combining existing refinery operations such as distillation with these new membrane units, cost savings through better purity, recovery, or through-put can be achieved. With these membranes acting as tight hyperfiltration devices, the permeation properties can be modeled through a solution-diffusion equation for molecular transport. Since these membranes also act as solvent resistant nanofiltration devices, unusual benefits can be found while practicing aromatics enrichment of refinery streams, including fractionation of different molecular weight species.