We have used atomically detailed simulations to predict the binary permeance of CH4/H-2 mixtures through defect-free (10,10) single walled carbon nanotubes acting as membranes at room temperature. Earlier simulations indicated that single-component gas diffusion in carbon nanotubes is extremely rapid compared to other known nanoporous materials. Our results show that this observation also holds for binary mixtures adsorbed in carbon nanotubes. As a result, carbon nanotube membranes are predicted to exhibit extremely high fluxes. Our results predict that carbon nanotubes are strongly selective for CH4 over H-2 when a mixture of these gases permeates through a membrane. Our results explore the performance of the membranes for mixtures of CH4 and H-2 as a function of the composition and total pressure of the feed gases and the total pressure drop across the membrane. (c) 2005 Elsevier B.V. All rights reserved.