Functionalization of single-walled carbon nanotubes (SWNTs) via atom-transfer radical addition in the presence of copper(I/II) redox systems allows for covalent attachment of a variety of different groups such as carboxylate and aryl. Modified SWNTs were characterized by ATR-IR, TG-MS, as well as Raman spectra, clearly indicating the formation of covalent bonding between organic groups and nanotubes.