Adsorption isotherms for hydrogen on single-walled nanotubes (SWNTs) subjected to various types of pretreatment have been measured by a tapered-element oscillating mass analyzer. Isotherms at room temperature over a range of pressures up to 48 bar have been measured. We demonstrate that activation of the SWNT samples by mild oxidation in CO2, followed by heat treatment in an inert atmosphere, increases the hydrogen adsorption capacity of the SWNT samples by about a factor of 3 at 48 bar. Computer simulations have been performed to model the adsorption isotherms. Bundles of homogeneous (all the same diameter) and heterogeneous (composed of a number of different diameters) nanotubes have been considered. Isotherms computed using a standard graphitic potential for the nanotubes give remarkably good agreement with the experimentally measured isotherms before activation with CO2. The effect of activation is modeled by independently increasing the nanotube spacing and the solid-fluid interaction potential. We find that nanotube spacing alone cannot account for the measured increase in adsorption capacity. Increasing the interaction potential gives isotherms that are qualitatively different from experiments, while a combination of increased nanotube spacing and increased solid-fluid potential gives rough agreement with experiments.