Advances in nanoporous membrane design with improvements in both chemical selectivity and high flux can directly benefit the fields of chemical separations, drug delivery, and wastewater remediation. Matching of pore size to that of the target molecules is critical to further advancement, because it will allow molecular sieving and forced interactions with chemically selective molecules bound to the pore. An array of aligned carbon nanotubes (CNTs) was incorporated across a polymer film to form a well-ordered nanoporous membrane structure. This membrane structure was confirmed by electron microscopy, anisotropic electrical conductivity, gas flow, and ionic transport studies. In principle, the inner cores of carbon nanotubes (CNTs) can enable fine control of pore dimension at the nanometer scale. During the CNT growth process, the nanotube size is set by the diameter of the catalyst particle, offering a practical route for pore diameter control through well-determined catalyst synthesis with nanometer-scale diameter dispersion. Carbon has also been deposited into porous alumina structures by a template method, making an aligned CNT membrane. The aligned growth of dense arrays of multiwalled CNTs has been demonstrated. Although the outer diameters had substantial variation (10 nm), the hollow inner-core diameter was well controlled at 2.3 nm, because more outer graphite walls were added with larger catalyst sizes. This inner-core diameter is in the size range of many proteins and other important biological macromolecules.