Hydrogen adsorption isotherms at 77, 87, and 298 K have been measured on three samples of single-wall carbon nanotubes. The highest adsorption capacity (1.58 wt % at 77 K, 1.15 wt % at 87 K, and 0.02 wt % at 298 K) at atmospheric pressure has been observed in a chemically activated sample (activated with KOH), which has hybrid porosity between a carbon nanotube material and a microporous activated carbon. According to CO2 adsorption at 273 K and density functional theory pore size distributions from N-2 adsorption, it is deduced that pores up to approximately 0.5-0.7 nm can adsorb hydrogen at ambient conditions. Isosteric heat of hydrogen adsorption has been calculated for the three samples, having initial values around 7-7.5 kJ mol(-1). It is concluded that the hydrogen adsorption capacity of carbon nanotubes depends both on the extent of their surface area and on the adsorption energy of the surface sites.