By using density functional theory calculations, it is found that the strongest interaction between NH3 and single-wall carbon nanotubes (SWNTs) is via its one H atom. The adsorption of NH3 on the metallic (7,7) SWNT turns out to be slightly stronger than that on the semiconducting (13, 0) SWNT. (c) 2005 Elsevier B.V. All rights reserved.