Pores of molecular dimensions can adsorb large quantities of gases owing to the enhanced density of the adsorbed material inside the pores(1), a consequence of the attractive potential of the pore walls, Pederson and Broughton have suggested(2) that carbon nanotubes, which have diameters of typically a few nanometres, should be able to draw up liquids by capillarity, and this effect has been seen for low-surface-tension liquids in large-diameter, multi-walled nanotubes(3). Here we show that a gas can condense to high density inside narrow, single-walled nanotubes (SWNTs), Temperature-programmed desorption spectrosocopy shows that hydrogen will condense inside SWNTs under conditions that do not induce adsorption within a standard mesoporous activated carbon, The very high hydrogen uptake in these materials suggests that they might be effective as a hydrogen-storage material for fuel-cell electric vehicles.