Since 1997 many research groups worldwide have investigated the hydrogen sorption of carbon nanotubes. A very wide range of data was published for the amount of hydrogen in carbon nanotubes starting at 0.39 mass% up to 67 mass%. We have investigated a large number of nanostructured carbon samples in an electrochemical half cell. The reversible amount of hydrogen stored in the nanotubes correlates with the specific surface area of the carbon samples. The maximum amount of hydrogen measured is 0.9 mass%.