This study reports experimental data of water adsorption at 303 K on single-wall carbon nanohorns (SWNHs). The analysis of the water adsorption isotherms supports a cluster-mediated model for filling the interstitial and intratube nanospaces in contrast to the monolayer formation observed for simple nonpolar molecules. The enthalpies of water immersion of SWNHs with closed and open nanohorns show a very week interaction between the water molecules and the hydrophobic carbon nanotubular structure; the observed specific enthalpy of immersion expressed per unit surface area is lower than the reported values for graphite. (C) 2002 Elsevier Science B.V. All rights reserved.