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The single-walled carbon nanotube sample (SWCNT) was treated with nitric acid. In addition, the SWCNT sample treated with nitric acid was heat-treated, and the hydrogen adsorption and desorption isotherms of these SWCNT samples were measured at 77 and 303 K over the hydrogen pressure range 0-3.5 MPa. The nitric acid treatment enhanced the amount of hydrogen adsorbed on SWCNT due to opening up the nanotubes and changing the bundle structure. The heat treatment further increased the hydrogen adsorption capacity of acid-treated SWCNT sample owing to the removal of the functional groups introduced by the nitric acid treatment.