Electronic states of nanodiamond (ND) were investigated by PEELS, XPS and CKVV Auger spectra. Parallel electron energy loss spectra (PEELS) show that the electrons inside of ND particles are sp(3) hybridized but there is a surface layer containing distinct hybridized states. The CKVV Auger spectra imply that the HOMO of the ND surface has a shift of 2.5 eV from natural diamond levels of sigma(p) up to the Fermi level. Hydrogen (H) treatment of natural diamond surface produces a chemical state indistinguishable from that of ND surfaces using CKVV. The ND electronic structure forms sigma(s)(1)sigma(p)(2)pi(1) surface states without overlapping of pi-levels. Surface electronic states, including surface plasmons, as well as phonon-related electronic states of the ND surface are also interesting and may also be important for field emission mechanisms from the nanostructured diamond surface. (C) 2003 Elsevier Science B.V. All rights reserved.