A simple, efficient, and practical method for the surface modification of multi-walled carbon nanotubes (MWCNTs) by nitric acid vapor is presented. X-ray absorption near edge structure (XANES) spectroscopy has been applied to study the temperature dependent evolution of the electronic structure of HNO3 vapor treated MWCNTs. After treatment the formation of oxygen containing functional groups was observed in C K-edge and O K-edge XANES spectra of MWCNTs. Moreover, the degree of oxidation made to MWCNTs can be easily controlled by changing treatment temperature. Compared to the conventional liquid HNO3 treatment, the advantages of this method should allow effective large-scale production of oxygenated MWCNTs for the applications. (C) 2011 Elsevier B.V. All rights reserved.