X-ray diffraction and X-ray absorption spectroscopy have shown that a material composed of two (Pu,Am)O2–x substoichiometric fluorite-type structures is obtained by sintering in moisturized Ar/H2. The Pu oxidation state is strictly +IV, whereas Am exhibits a +III/+IV mixed valence. Thus, the Am–O system and particularly the Am oxidation state play a major role in the sintering behavior of the Pu–Am–O system. The heating of this compound in air leads to the formation of a fluorite Pu0.80Am0.20O1.98 solid solution. The transition from a one-phase to a two-phase structure takes place at a temperature lower than 800 °C. The results were compared with previous studies on Am-bearing UO2 and ThO2, which have evidenced different behaviors.