Single wall carbon nanohorns (SWNHs) were treated in vacuum at different temperatures of 473 to 1073 K. The nanostructural change due to the heat-treatment vas studied by adsorption of N-2 at 77 K and H2O at 303 K. The determined particle density showed that gas is not adsorbed in internal pores, but in interstitial pores. The high temperature treatment (HTT) in vacuo changed water adsorption, but it gave almost no influence on N-2 adsorption. The maximum nanopore volume from H2O adsorption was observed at 673 K, indicating the interstitial nanopore change due to a local orientational change of SWNH particles. (C) 2004 Elsevier B.V. All rights reserved.