In this paper, we determine for the fjrst time the electronic, structural and energetic properties of U 1−y Am y O 2 mixed oxides in the entire range of Am content using the generalized gradient approximation (GGA) +U in combination with the special quasirandom structure (SQS) approach to reproduce chemical disorder. This study reveals that in U 1−y Am y O 2 oxides, Am cations act as electron acceptors, whereas U cations act as electron donors showing a fundamental difference with U 1−y Pu y O 2 or U 1−y Ce y O 2 in which there is no cation valence change in stoichiometric conditions compared to the pure oxides. We show for the fjrst time that the lattice parameter of stoichiometric U 1−y Am y O 2 follows a linear evolution which is the structural signature of an ideal solid solution behavior. Finally, using two approaches (SQS and parametric), we show by assessing the enthalpy of mixing that there is no phase separation in the whole range of Am concentration.