In this work, we report an investigation by means of density functional theory (DFT) of the adsorption of hydrogen and the separation of hydrogen-carbon monoxide mixture in an isolated single-walled carbon nanotube. The theory is based on a perturbative construction of free energy functional for inhomogeneous pure fluid and binary fluid mixture. The reformulated Rosenfeld's fundamental-measure theory using the excess Helmholtz energy density from the Boublik-Mansoori-Camahan-Starling-Leland equation of state proposed by Yu and Wu (J. Chem. Phys. 117 (2002) 10156) is applied to represent the pure and binary hard-sphere repulsive interaction, and Weeks-Chandler-Andersen perturbation theory is used to build the attractive contribution. The density profiles in three sizes of tubes at 300 K and reduced bulk density from 0.2 to 0.7 for pure hydrogen and hydrogen-carbon monoxide mixture are obtained. The theoretical calculations are in good agreement with the simulation results in this work and other data available in literature. The adsorption of hydrogen and the selectivity of hydrogen-carbon monoxide mixture are predicted from DFT and the adsorption characteristics of the isolated cylindrical wall is discussed. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.