The hydrogen storage capacities of palladium- and vanadium-doped carbon nanotubes (CNTs) at room temperature were investigated using the Sieverts apparatus. The storage capacity of Pd- and V-doped CNTs at 2 MPa were found to be 0.66 and 0.69 wt%, which are nearly 30% more than that of the pristine-CNTs. Also, the doped-CNTs exhibited faster initial hydrogen adsorption kinetics when compared with the pristine samples. The compensation of hydrogen storage by the metal particles in the doped-CNTs reveals that metal particles enhance the storage capacity via the spill-over mechanism. The re-adsorption studies of previously adsorbed samples suggested that nearly 70-85% of the spilled hydrogen occupies the physisorption binding sites, such as external-walls or groove-sites of CNTs. (c) 2005 Elsevier B.V. All rights reserved.