Adsorption of deuterium gas in single-wall carbon nanotubes (SWNTs) with closed end, doped with potassium was simultaneously studied by adsorption isotherms and neutron diffraction in the range 18-300 K. Neutron diffraction has shown that doped SWNTs are only slightly intercalated by K ions so that empty cavities are preserved in between the tubes. At room temperature, the chemisorption of deuterium molecules was not observed in doped SWNTs bundles, but only in the KC8 graphite intercalation compound impurities. At low temperature, the isotherms analysis and neutron diffraction experiments have shown that D-2 molecules are physisorbed in doped SWNTs in the free interstitial voids. (C) 2003 Elsevier B.V. All rights reserved.