High-purity N-doped double-walled carbon nanotubes (DWNTs) were synthesized by chemical vapor deposition, and investigated by synchrotron X-ray photoelectron spectroscopy (XPS). As the photon energy increases, the N content increases up to 3 at.%, indicating that the inner wall contains the higher N content than the outer wall. The graphite-like and pyridine-like N structures exist as about 1:1 ratio. The self-consistent charge-density-functional-based tight-binding calculation shows that the N-doping of the inner wall yields the more stable DWNT than that of the outer wall, which supports the XPS result. (c) 2005 Elsevier B.V. All rights reserved.