Multiwalled carbon nanotubes (MWNTs) were surface photo-oxidized at room temperature with UV (253.7 and 184.9 nm) and vacuum UV (VUV) (106.7 and 104.8 nm) radiation. X-ray photoelectron spectroscopy (XPS) showed rapid UV photo-oxidation during the first hour of treatment and then an increase that was directly proportional to the time of treatment up to 4 h where the oxygen concentration was 7.5 at%. VUV photo-oxidation resulted in an oxygen concentration up to 9.5 at% with exposure time for the initial 2 h of treatment. Beyond 2 h, the oxygen concentration decreased with exposure to VUV photo-oxidation. Curve fitting of the XPS C-1s spectra revealed mainly C-O-C, as ether or epoxy functional groups, with the presence of C=O, O-C=O and O=C-O-C=O or carbonate moieties. SEM micrographs showed no apparent effect on the structure or appearance of the MWNTs as expected from surface modification. Gas-phase UV and VUV photo-oxidation effectively functionalize MWNTs for potential adhesion improvement without resulting in liquid waste as from the traditional bulk processing method involving acidic oxidation.