We investigated the electronic state of an (Am,U) mixed oxide with the fluorite structure using the all-electron full potential linear augmented plane wave method and compared it with those of Am 2 O 3 , AmO 2 , UO 2 , and La 0.5 U 0.5 O 2 . The valence of Am in the mixed oxide was close to that of Am 2 O 3 and the valence of U in the mixed oxide was pentavalent. The electronic structure of AmO 2 was different from that of Am 2 O 3 , particularly just above the Fermi level. In addition, the electronic states of Am and U in the mixed oxide were similar to those of trivalent Am and pentavalent U oxides. These electronic states reflected the high oxygen potential of AmO 2 and the heightened oxygen potential resulting from the addition of Am to UO 2 and also suggested the occurrence of charge transfer from Am to U in the solid solution process.