Hydrogen isotherms on a variety of single-walled carbon nanotube (SWNT) samples were measured using a differential pressure adsorption apparatus, which provides highly accurate data. A number of these SWNT samples were modified by a non-destructive cutting process, which reduced the aspect ratio of the nanotube bundles by two orders of magnitude. There were no apparent differences in the microporosity of SWNT as a function of aspect ratio. The adsorption of helium on SWNT is shown to be non-negligible and results in artificially low hydrogen capacities using conventional adsorption methodology. With no accounting for helium adsorption, the hydrogen adsorption results show that cut and uncut SWNT have similar hydrogen capacities of <1 wt% at 25 °C and pressures up to 110 bar. However, an analysis of hydrogen capacity versus N2 Brunauer–Emmett–Teller surface area suggests that there is an enhanced heat of adsorption of hydrogen for SWNT versus activated carbon.