The characterization of chemically modified carbon nanotubes has been achieved using C-13 nuclear magnetic resonance (NMR) spectroscopy. Amino-functionalized multiwall carbon nanotubes (MWNT-NH2) were reacted with a C-13 terephthalic acid. Covalent attachment of the acid to the nanotubes is confirmed by C-13 NMR and 2D NMR through the presence of amide grouping resonances.