The electronical and structural properties of Th0.80Am0.20O2-x materials have been studied by the coupling of X-ray diffraction and X-ray absoriotion spectroscopy techniques. A substoichiometric fluorite (Th0.80Am0.20O1.90)-Am-IV-O-III solid solution is found following sintering in moisturized Ar-H-2. In contrast, heating of this sample in air leads to a nondefective fluorite (Th0.80Am0.20O2.00)-Am-IV-O-IV solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO2 to accommodate tri- and tetravalent actinides in the fluorite structure assessed.