A series of Co-doped ZnO are obtained by thermal decomposition of the oxalate precursors. Co ions are in the 2+ formal oxidation states and substituted Zn in ZnO. The band gap energy E-g and the absorption threshold E-CT of the charge-transfer transition related to Co2+ decrease with the increasing Co concentration x, but the energy differences between them are almost invariable for all Co-doped ZnO. E-g and E-CT reduction with x are mainly due to the valence band raised, and the energy of the acceptor-type ionization levels relative to the conduction band edge is independent of Co content. (C) 2008 Elsevier B.V. All rights reserved.