Abstract: A novel membrane filtration system for separating gases of closely spaced molecular sizes incorporates a uniform, pore free, ultrathin membrane of a rubbery polymer which functions to transport different gases at different rates by sorption effects. The membrane is formed on a microporous substrate without convective pores by preventing incipient polymerization before rapid curing in a deposition process. Employing porous but thin structural supports for the membrane/substrate layers, cells, modules and systems can be arranged for cost effective improvement of the thermal quality of nitrogen-contaminated methane in natural gas.