The lattice parameters of (Am0.50U0.50)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2−x were determined by powder X-ray diffraction with Cu Kα radiation. In addition, the lattice parameter functions of (AmyU1−y)O2−x (0.00<x<0.25, 0.00<y<0.50) were evaluated using models of (Am3+yU4+1−2yU5+y)O2 and (Am3+yU4+1−y)O2−y/2 based on the results of X-ray diffraction and the ionic radii of Am3+, U4+, and U5+. In order to confirm the valence state of Am and U in (AmyU1−y)O2−x, the X-ray absorption near-edge structure measurements were performed in the transmission mode at the Am-LIII and U-LIII absorption edges of (Am0.50U0.50)O2.0, (Am0.50U0.50)O2−x, and UO2.0.