Recent theoretical work by Sholl and coworkers has predicted that carbon nanotubes, if used as membranes, have the flux/selectivity properties that far exceed those of any other known inorganic or organic material. To verify this prediction, we have fabricated nano-composite membranes consisting of single-walled carbon nanotubes embedded in a poly(imide siloxane) copolymer and evaluated their transport properties. While the siloxane segment enhanced the interfacial contact, the polyimide component imparted mechanical integrity. A poly(imide siloxane) was synthesized using an aromatic dianhydride, an aromatic diamine and amine-terminated PDMS for the siloxane block. The weight percent of PDMS was determined to be 41 using H-1-NMR. Permeability measurements of He showed drops in permeability with the addition on close-ended CNTs. This large drop in permeability of He suggests that the copolymer adhered well to the CNTs and that the prepared CNT MMMs were defect free. However, the permeability of O-2, N-2 and CH4 increased in proportion to the amount of open-ended CNTs in the polymer matrix. This suggests that CNTs offer an attractive additive for universally enhancing the gas permeability of polymers.