Book excerpt: The thermodynamic data for unirradiated dioxides in the U- ThPu-O system are briefly reviewed. Calculations of oxygen release during fission were made and were found to agree qualitatively with experimental measurements. Theories were developed for prediction of mass transport rates in oxide particles; these included control by either CO-- CO/sub 2/ diffusion, reactions such as disproportionation of carbon monoxide, or solid-state diffusion through the kernel. Methods were devised for the determination of the general type of rate- controlling mechanism from the quantitative analysis of inreactor and out-of- reactor experiments. Application of these methods to an in-pile experiment indicated that CO-- CO/sub 2/ diffusion did not control the rate of UO/sub 2/ or (Th,U)O/sub 2/ migration in BISO particles; instead, control by solid-state diffusion was indicated. (54 references) (auth).