The permeability performance of mixed matrix carbon-nanotube/polymer membranes (see figure) is estimated by using the finite element method. The universal approximation of perfectly permeable nanotubes appears appropriate for predicting the overall rates of single gas solute transport. Based on direct finite element predictions, a set of simple design equations demonstrate that nanotube/polymer membranes can favorably combine the high-flux performance of nanotubes with the intrinsic selectivity of a polymer matrix.