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The valences of Cu and Ti ions in CaCu 3 Ti 4 O 12 (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 Cu 2þ to Cu 1þ . We also present the evidence concerning the creation of the excess holes in the oxygen 2p levels resulting in a 3d 9 L copper configuration in CCTO, due to the development of Cu vacancies and related to the reduction of Ti 4þ to Ti 3þ .The nature of the colossal dielectric constant of CaCu 3 Ti 4 O 12 (CCTO) has been under intensive exploration in the past decade. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] After thorough investigation, the extrinsic barrier mechanism is nowadays considered to be responsible for its anomalous dielectric behavior. The framework of the barrier model of CCTO consists of semiconducting and insulating regions. While the latter has been identified, 10,12,13,17 the enigma concerning the very origin inducing the semiconductivity in the grain interiors of CCTO still remained unresolved. Nevertheless, a scenario concerning the compositional variation had been developed, 9,12 where Cu valence is the key element relevant to the reduction of Ti 4þ to Ti 3þ . The valences of ions in oxides were mainly determined by X-ray photoelectron spectroscopy (XPS) known to cause less radiation damage to the sample surface than Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). Sometimes, however, XPS causes sample degradation during the analysis. [18][19][20][21][22] Various explanations have been advanced to explain the reduction of CuO during measurements, such as photon, 18-20 thermal, 21 and vacuum effects. 21,22 Though considerable Cu 1þ ions had surprisingly been observed in XPS measurement of CCTO, they actually would result in structure instability due to the charge imbalance and lattice distortion. In view of significant variation of the valences of copper species in vacuum-annealed copper oxides, 19 we are promoted to reassess the valences of Cu and Ti ions by XPS. The significant effect of the room-temperature vacuum outgassing during XPS measurement on determining the valences of copper and titanium species in CCTO is also presented. ExperimentalSingle-phase CCTO was synthesized by solid state reaction. The samples were sintered at 1100 C in air for 20 h. The XPS studies were carried out employing an Al Ka monochromatic X-ray source. The room-temperature vacuum outgassing time of the samples was counted after reaching the desired vacuum pressure (<10 À6 Pa) (about 10 min). The shift of core-level spectra was calibrated by the C 1s peak of "adventitious" carbon located at 284.8 eV. The Lorentzian-Gaussian ratios in peak fitting have been constrained between 10-90 and 20-80. 23 Shirley backgrounds were used.Results and Discussion Figure 1 shows the variation of room-temperature vacuum...