Xe-129 and Xe-131 nuclear magnetic resonance (NMR) spectroscopy was used to study the adsorption of xenon gas on as-produced single-walled and multi-walled carbon nanotubes. Overall, the adsorption was weak, with slightly stronger interaction between xenon and multi-walled nanotubes. Temperature-dependent spectra, relaxation times, line widths, and signal intensities provide evidence that xenon forms a multilayer bulk phase rather than a homogeneous surface coating. The estimated adsorption energy of 1.6 kJ/mol is significantly lower than 23 kJ/mol predicted for monolayer adsorption but is in keeping with the Xe-Xe attractive potential. Xenon preferentially adsorbs on metallic particles in single-walled tubes, while defects are the nucleation sites for the stronger adsorption on multi-walled tubes.