The electrochemical storage of hydrogen on a range of carbon nanotubes has been investigated using electrochemical techniques and in situ Raman spectroelectrochemistry. An aggregated, single-walled nanotube sample was found to have the highest apparent storage capacity of 0.22 wt % (3 M KOH). Raman spectroelectrochemistry confirmed that no irreversible structural changes occur upon charging. The storage was found to be primarily due to the sorption of H2 gas in the pores of the nanotube aggregate, combined with some chemisorption on the amorphous carbon impurities in the sample. It is, therefore, concluded that the observed storage capacity is due to the small pores and the presence of carbonaceous impurities. © 2010 American Chemical Society.