Interaction of carbon monoxide with armchair single-walled carbon nanotubes (SWCNTs) of various diameters was investigated using the B3LYP density functional theory (DFT) level. For both the external and internal cases, the effect of molecular orientation on the adsorption process was studied. The adsorption energies, equilibrium distances, HOMO-LUMO energy gaps, partial charges, and natural atomic orbital occupancies of the interacting atoms were also studied. It was found that physisorption of molecular CO via the oxygen head was stronger than that via the carbon head, and that the most favorable adsorption angle between the molecular axis and x-axis of the tubes for all the armchair nanotubes was 135 degrees. Furthermore, stability of the tube molecule system for adsorption of a CO molecule on the outer wall of a nanotube decreases as the tube diameter increases, regardless of the adsorption angle. In contrast, CO penetration into a nanotube depends on the adsorption angle. The results show that molecular CO could be adsorbed on the inner walls of the armchair nanotubes provided that the molecular axis and the main axis of the nanotubes are parallel to each other. (C) 2010 Elsevier B.V. All rights reserved.