Multi-walled carbon nanotube (MWNT) paper was surface modified with gaseous oxygen and chlorine atoms, and analyzed by X-ray photoelectron spectroscopy (XPS). The oxygen atoms were produced by a low-pressure microwave (MW) plasma discharge of an Ar-O-2 mixture and the results were compared to a previous study using VUV photo-oxidation. Exposure to VUV photo-oxidation (lambda = 104.8 and 106.7 nm) showed approximately the same level of oxidation (ca. 7.4 at% O) as the MW results (ca. 6.3 at% O) that were conducted in the absence of radiation from the plasma. Oxygen atoms reacted with the pi-conjugation of the MWNTs to primarily produce the epoxide/ether groups. Chlorine atoms, which were generated by UV photolysis of Cl-2, reacted more readily than chlorine molecules with the nanotubes to yield saturation levels of ca. 13 at% Cl. (C) Koninklijke Brill NV, Leiden, 2012