The self-diffusion of 238Pu was measured in an oxicarbide (U,Pu)(C,O) and a carbonitride (U,Pu)(C,N). The activation enthalpies were 447 and 347 kJ·mol−1, respectively. The carbonitrides were confirmed to fall into three classes: carbide-like compositions with less than 30% nitrogen in the metalloid lattice, nitride-like composition with more than 70% nitrogen and with reduced atomic mobilities, and carbonitrides with ∼50% nitrogen showing an intermediate behavior. The oxicarbide showed diffusion coefficients slightly larger than those of pure carbides.