Gas-solid interactions between hydrogen and single-walled carbon nanotubes (SWNTs) were investigated using highly purified SWNTs. The activation energy of hydrogen desorption, measured to be 0.21 eV, indicates that hydrogen is physisorbed in the, pores and that inter-tube pores have an adsorption potential of about -0.21 eV, which induces hydrogen physisorption at ambient, temperature. The total amount of adsorbed hydrogen; about 0.3 wt% at 9 MPa shows that 38% of the inter-tube sites are occupied.. These findings are interpreted in terms of the chemical potential of hydrogen and the adsorption potential of the inter-tube pores. (C) 2002 Elsevier Science B.V. All rights reserved.