MWNTs were photo-oxidized in quartz boats using low-pressure Hg lamps and emission downstream from Ar microwave plasmas which are primarily atomic line sources of 253.7 and 184.9 nm UV, and 106.7 and 104.8 nm vacuum UV (VUV) radiation, respectively. X-ray photoelectron spectroscopy (XPS) showed rapid oxidation during the first hour of UV 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 VUV exposure due to a larger rate of de-oxidation than oxidation at the surface. Curve fitting of the XPS C1s spectra revealed mainly the C-O-C functional group with the presence of C=O, O-C=O and O=C-O-C=O moieties. SEM micrographs showed no apparent effect on the structure or appearance of the MWNTs as expected for surface modification.