Single Langmuir-Blodgett bilayers derived from 5,11,17,23,29,35-hexakis[(N,N,N-trimethylammonium)-N-methyl-37,38,39,40, 41,42-hexakis-n-hexamedecyloxy-calix[6]arene hexachloride (1), which have been ionically cross-linked (i.e., "glued together") with poly(acrylic acid), have been found to exhibit He/CO2 permeation selectivities as high as 150. This degree of selectivity, for a membrane that is less than 6 nm in thickness, is without precedent. In principle, materials of this type could lead the way to improved membranes for hydrogen purification.