The valences of Cu and Ti ions in CaCu3Ti4O12 (CCTO) are reassessed by characterizing the spectra of Cu(2p(3/2)), O(1s), and Ti(2p(3/2)) using X-ray photoelectron spectroscopy (XPS). The structure instability of CCTO leads to easy expulsion of oxygen from lattice during vacuum outgassing in XPS measurement, wherein the created oxygen vacancies reduce Cu2+ to Cu1+. We also present the evidence concerning the creation of the excess holes in the oxygen 2p levels resulting in a 3d(9)(L) under bar copper configuration in CCTO, due to the development of Cu vacancies and related to the reduction of Ti4+ to Ti3+. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610225] All rights reserved.