Process for radiolabelling of carbon nanotubes comprising surface carboxyl groups, comprises substitution of surface carboxyl groups by radiolabelled nitrile groups to react the nanotubes with a carbon 13 or carbon 14 radiolabelled cyanide in presence of palladium catalyst and an organic solvent for producing radiolabelled carbon nanotubes comprising carbon 13/carbon 14 labeled nitrile groups, and hydrolysis of the nitrile group to react the nanotubes with an acid/base in an organic solvent/an aqueous organic mixture for producing the radiolabelled carbon nanotubes comprising carboxyl groups. Process for radiolabelling of carbon nanotubes comprising one or more surface carboxyl groups, comprises substitution of surface carboxyl groups by radiolabelled nitrile groups to react the carbon nanotubes with a carbon 13 or carbon 14 radiolabelled cyanide in presence of palladium catalyst and an organic solvent for producing radiolabelled carbon nanotubes comprising one or more carbon 13 or carbon 14 labeled nitrile groups, and hydrolysis of the nitrile group to react the carbon nanotubes with an acid or base in an organic solvent or an aqueous organic mixture for producing the radiolabelled carbon nanotubes comprising one or more carboxyl groups in which the carbon atom carrying the acid function is carbon 13 or carbon 14. An independent claim is included for the radiolabelled carbon nanotube obtained by the above process.
