Carbon nanotubes functionalized with sulfonated polyether-ether-ketone are investigated using solid state nuclear magnetic resonance. Carbon and proton NMR experiments of the sulfonated polymer chains covalently grafted on the nanotube surface reveal a distribution of diamagnetic shifted lines. These experimental results can be interpreted at the molecular level in terms of magnetic ring currents originating from the surface of the graphitized wall of the nanotubes in agreement with recent theoretical investigations. These features can potentially be used to track the structural modifications, which take place during the functionalization of carbon nanotubes. © 2009 American Institute of Physics.