An experimental investigation on the phase relation of PuO 2−x with addition of 9% americium (Am) was carried out by means of X-ray diffraction analysis, ceramography and differential thermal analysis. Solid solutions of (Pu 0.91 Am 0.09 )O 2−x with different O/M ratios ranging from 1.90 to 2.00 were prepared. The phase transition temperatures of single phase to two phases at high temperatures were slightly lower than those of AmO 2−x . The phase relation of (Pu 0.91 Am 0.09 )O 2−x seemed to have different trends which appeared above and below a certain O/M ratio. Consideration of the valence states of Pu and Am in the solid solution could lead to a reasonable interpretation for the phase relation.