We investigated the electronic structure of double-walled carbon nanotube (DWNT) bundles by variable-energy X-ray photoelectron spectroscopy (XPS). High-purity DWNT bundles were synthesized using MgO-supported Co-Mo catalysts by chemical vapor deposition of CH4. As the photon energy of XPS increases from 360 to 1265 eV, the width of C Is peaks becomes broader, probably due to the van der Waals interaction between the nanotubes in bundles. The XPS valence band analysis suggests that the present DWNT bundles would have more delocalized graphite-pi electrons than the multiwalled nanotubes. (C) 2004 Elsevier B.V. All rights reserved.