Considerable effort has been expended to synthesize the carbides, nitrides, and carbonitrides, of uranium, Plutonium, and their solid solutions by the carbothermic reduction of oxide-carbon mixtures. Experimental results reported in the literature for the U-C-O-N system indicate that one of two limiting mechanisms controls the rate of the conversion reaction: These mechanisms are solid-state diffusion within the individual reactantproduct particles, and removal of CO from the entire reacting system. This reported information is used to analyze the use of a fluidized bed for carbothermic conversion in the vicinity of 1500°C in the U-C-O-N system and to approximate the rate of conversion in the U-Pu-C-O-N system.