We have studied the work function and density of states (DOS) of multiwall carbon nanotubes (MWNTs) using ultraviolet photoelectron spectroscopy (UPS). Raw MWNTs were purified by successive sonication, centrifugation, sedimentation, and filtration processes with the aid of a nonionic surfactant. The purified MWNTs showed a slightly lower work function (4.3 eV) than that of highly oriented pyrolytic graphite (4.4 eV). Effects of three different oxidative treatments, air-, oxygen plasma-, and acid-oxidation, have also been studied. It was found that oxidative treatments affect the DOS of valence bands and increase the work function. X-ray photoelectron spectroscopy (XPS) measurements have suggested that gas-phase treatment preferentially forms hydroxyl and carbonyl groups, while liquid-phase treatment forms carboxylic acid groups on the surface of MWNTs. These surface chemical groups disrupt the pi-conjugation and introduce surface dipole moments, leading to higher work functions up to 5.1 eV. We expect the information on the work function of the MWNTs to be of importance to the development of electronic or optoelectronic applications.