All available oxygen potential-temperature-composition data for the calcium fluorite-structure ** phase were retrieved from the literature and utilized in the development of a binary solid solution representation of the phase. The data and phase relations are found to be best described by a solution of [Pui^C^] and [PuO 2 ] with a temperature dependent interaction energy. The fluorite-structure is assumed to be represented by a combination of the binaries' and CUO^^, and thus treated as a solution of [Pu^/302], [PuO 2 ], [UO 2 ], and either [U 2 0i,.5] or [U3O7]. The resulting equations well reproduce the large amount of oxygen potential-temperature-composition data for the mixed oxide system, all of which were also retrieved from the literature. These models are the first that appear to display the appropriate oxygen potential-temperature-composition and phase relation behavior over the entire range of existence for the phases.