Activities directed toward establishment of the composition limits for single-phase face-centered cubic structures in the UO/sub 2/ - ZrO/ sub 2/-CaO and UO/sub 2/-CeO/sub 2/ systems are reported. Results of efforts to determine the physical, chemical, corrosion, and thermal properties of selected single-phase cubic structures in the two ceramic fuel systems are also included. (J.R.D.)