Reflectivity spectra of nine rare-earth sesquioxide (R 2 O 3 ; R = La, Pr, Nd, Sm, Eu, Er, Tm, Yb and Lu) single crystals have been measured in the photon energy range from 0.5 to 50 eV at 300 K. The fundamental optical absorption has been clarified in connection with the electronic structure of R 2 O 3 . The absorption due to an intra-atomic optical transition of R4f → R5d6s was observed in the same energy region as the charge transfer excitation of O2p → R5d6s in the energy range of 6-15 eV. The absorption structure of the R4f → R5d6s transition and the R5p → R5d6s transition are explained by using the crystal field splitting of the 5d band and the multiplet structure of the 4f n−1 final state in photoemission process. Fine structures due to the absorption of the R4f → O3s transition in the energy region of the O2p → O3s absorption were observed and also can be explained by the 4f n−1 final state.Conductive materials with 3d, 4f and 5f electrons have attracted attention because of the interesting physical properties which originate from the interaction between carriers and magnetic moments. So-called heavy fermions and valence fluctuation states are due to the interaction. In the case of insulating materials, valence fluctuation states also appear in spite of the absence of carriers because localized magnetic moments strongly hybridize with valence and conduction bands. The typical strongly hybridized insulator is CeO 2 . 1) All rareearth oxides including CeO 2 are insulators with 4f electrons. 2) Many of the rare-earth oxides are sesquioxides (R 2 O 3 , R means rare-earth) with trivalent rare-earth ion. However, some materials take different oxidation states. EuO 3) and Eu 3 O 4 4) are such materials, which have already been studied by optical and the other observations. Commercially available powders of Pr and Tb oxides are Pr 6 O 11 and Tb 4 O 7 , respectively. In these materials, the rare-earth ions take intermediate valence between 3+ and 2+ (mixed valence state). The origin of the mixed valence may be mixing between 4f electrons and valence band electrons. Previously we have investigated the electronic structure of three kinds of Pr oxides with different oxidation states using reflectivity and photoelectron spectroscopies and the results have already been published. 5) These results indicate different degrees of mixing. To understand more clearly the electronic structure of the mixed valence oxides, the electronic structure of normal insulators of R 2 O 3 should be clarified first of all. In the present study, we have in- * Present address: vestigated the electronic structure of R 2 O 3 by optical measurements.In previous studies of R 2 O 3 and related materials, optical investigations of R 2 O 3 have been mainly done on the optical phonon structure, 6-9) antiferromagnetic phase 10-13) and 3d → 4f absorption edges. 14, 15) The dipole forbidden f → f transition in Yb 2 O 3 was investigated by electron energy loss spectroscopy. 16) The transition originates from the crystal field splitting of Yb 4f 13 state...

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