We have studied the electronic structure of the spinel-type compound CuIr 2 S 4 using x-ray photoemission spectroscopy (XPS). CuIr 2 S 4 undergoes a metal-insulator transition (MIT) at 226 K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of 0:15 eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir 3 :Ir 4 1:1. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr 2 S 4 is very robust against photoexcitation in contrast to Cs 2 Au 2 Br 6 which shows photo-induced valence transition.The spinel-type compound CuIr 2 S 4 has been attracting much interest because of its first-order metal-insulator transition (MIT) at T MI 226 K accompanied by the loss of localized magnetic moments [1][2][3][4][5][6]. Since the valence state of the Cu ion is Cu , an ionic configuration of Cu Ir 3 Ir 4 S 2ÿ 4 is expected in the insulating phase [7][8][9][10][11][12]. A recent structural study [13] indicates that the cubic spinel structure of CuIr 2 S 4 becomes tetragonally elongated along the c axis and that octamers of Ir 3 (S 0) and Ir 4 (S 1=2) are formed below T MI . In the Ir 4 octamer, the Ir 4 ions are dimerized in two directions (see Fig. 1). Croft et al. have found a dramatic electronic structural change above the Fermi level (E F ) across the MIT using x-ray absorption spectroscopy [4]. They have proposed that the low-temperature structure can be decomposed into onedimensional chains and that the dimerization due to charge and orbital ordering, i.e., the charge density wave formation along the chain direction, is responsible for the electronic structural change [4]. Very recently, it has been proposed that the dimerization in CuIr 2 S 4 and MgTi 2 O 4 [14] can be understood as an orbitally driven Peierls transition [15]. The experimental and theoretical studies indicate that the electronic structure of CuIr 2 S 4 itself is very exotic and interesting. Another interesting point is that the resistivity of CuIr 2 S 4 is dramatically reduced by x-ray or visible light irradiation in the insulating phase [16 -18]. It has been proposed that the photoexcitations break the Ir 3 =Ir 4 charge ordering and induce metallic conductivity.The nature of charge ordering and the effect of light irradiation are still controversial, partly because of the difficulty in the photoemission measurement. Photoemission spectroscopy is a powerful technique to investigate electronic states below E F although it is a surface sensitive method and clean surface must be prepared to obtain precise information. In previous photoemission studies of polycrystalline CuIr 2 S 4 , the Ir 4f core-level spectrum was reported to show no spectral change across the MIT [11,12] and no evidence of charge ordering was obtained. The Ir 4f photoemission data can provide ...