Carbon nanotubes, mostly single- and double-walled, are prepared by a catalytic chemical vapor deposition method using H-2-CH4 atmospheres with different CH4 contents. The maximum hydrogen storage at room temperatures and 10 MPa is 0.5 wt %. Contrary to expectations, purification of the carbon nanotube specimens by oxidative acid treatments or by heating in inert gas decreases the hydrogen storage. Decreasing the residual catalyst content does not necessarily lead to an increase in ASH. Moreover, increasing the specific surface area does not necessarily increase the hydrogen storage capacity. There seems to be a correlation between the pore volume at low pore diameters (<3 nm) and the hydrogen storage capacity. Contribution from nanoscale disordered carbon to the hydrogen storage cannot be ruled out.