Hydrogen adsorption on carbon nanotubes was studied with H-1 and H-1 NMR spectroscopy. A total of eight single- and multiwalled carbon nanotube samples provided by various manufacturers and synthesized by different production methods were investigated before and after an oxidative purification procedure. NMR data are corroborated by TEM, ICP-MS, and ESR spectroscopy. The presence of residual metal catalyst particles causes considerable broadening of the spectra. While susceptibility broadening and dipolar interactions often dominate the H-1 NMR spectra, quadrupolar interactions lead to additional broadening in the H-1 NMR. Smaller line widths in the H-1 NMR data allowed for free hydrogen gas and adsorbed hydrogen to be distinguished. The adsorption is fast and reversible and must be described as physisorption. Hydrogen uptake capacities at a pressure of 1480 kPa and at ambient temperature were estimated to be about 0.1 wt %.