Abstract The polycrystalline thin film of Bi 1− x Y x O 1.5 ( x = 0.3) has been prepared by r.f.-sputtering method and was annealed under various oxidation conditions for the introduction of excess oxygens. In Bi 0.7Y 0.3O 1.5+δ, as the preparation condition became oxidative, the band gap decreased from 3.0 to 2.1 eV, indicating the formation of Bi 5+. In pseudo-perovskite crystal, BaBiO 3 thin film which was prepared for comparison, the intense and broad absorption band with the absorption coefficient of 10 5 cm −1 located at 2.2 eV was observed. The absorption bands observed in both oxides were ascribed to the charge transfer transitions from Bi 3+ 6 s 2 to Bi 5+ 6 s 0. However, they had different intensities and energy positions. These transitions were classified into two types from the point of view of lattice vibration of anions between mixed-valence cations. In perovskite type in which ( BiO 6 2 ) octahedra are linked by sharing corner, the vibration of apical oxygens was expected to enhance the charge transfer transition.